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OT: S10 with twittering sound, probably vacuum hose somewhere. can't find it.

JST

Diamond
Joined
Jun 16, 2001
Location
St Louis
The 2000 S10 is making a twittering sound that I associate with vacuum, because it corresponds with certain engine speeds and throttle positions. Only does a short chirp when the accelerator is worked at idle, but definitely does it under load when driving. Does not do it when floored, does it when starting up from a stop (clutch vehicle, 4 cylinder flex fuel engine)

Oddly, it does this only after warming up. It does not do it when cold. But once warmed up it is consistent. The warm up issue seems to associate it with the closed loop emissions system, but I have a more general Haynes manual, which does not have a solid year-by-year diagram of the system.

One odd part is that I can understand a straight whistle sound, but the sound is really a twittering sound, with a low frequency variation to it. Similar to whistles with the little ball in them to add a "vibrato". Heard both inside and outside. (I have eliminated the chance of POTUS being in there somewhere tweeting). The net result is almost like the sound of a knock, but does not occur when a knock should occur.

Everything that I know about that is driven by vacuum seems to be working, and the vehicle is in good shape, worth working on. I cannot find any leaks or cracks in visible hoses.

Any ideas?
 
The 2000 S10 is making a twittering sound that I associate with vacuum, because it corresponds with certain engine speeds and throttle positions. Only does a short chirp when the accelerator is worked at idle, but definitely does it under load when driving. Does not do it when floored, does it when starting up from a stop (clutch vehicle, 4 cylinder flex fuel engine)

Oddly, it does this only after warming up. It does not do it when cold. But once warmed up it is consistent. The warm up issue seems to associate it with the closed loop emissions system, but I have a more general Haynes manual, which does not have a solid year-by-year diagram of the system.

One odd part is that I can understand a straight whistle sound, but the sound is really a twittering sound, with a low frequency variation to it. Similar to whistles with the little ball in them to add a "vibrato". Heard both inside and outside. (I have eliminated the chance of POTUS being in there somewhere tweeting). The net result is almost like the sound of a knock, but does not occur when a knock should occur.

Everything that I know about that is driven by vacuum seems to be working, and the vehicle is in good shape, worth working on. I cannot find any leaks or cracks in visible hoses.

Any ideas?

Sounds like bearings & electric clutch plate chirp in the AC compressor drive to me. Or so it were on my '91 S15, which is just an S10 with two extra lockwashers and a "GMC" badge.

You'll now tell me your cheaper S10 doesn't HAVE AC, so... start disabling one hose at a time 'til you find it. Odds are it will be in one of the silly doo-dads that live off the vacuum system, not the hoses themselves.

My EGR valve was converted to "all manual" for example. One position for emissions inspection one day every two years. The other for daily-driver the rest of the time.

:)
 
Could be the idle air bypass valve. The throttle blades are shut at idle, and the air goes through a different path. It could have some fouling.
 
The AC clutch...... why would it seem to be engine vacuum related?

Could be the idle air bypass valve. The throttle blades are shut at idle, and the air goes through a different path. It could have some fouling.

Sounds at least related to vacuum. Would that cause the noise when the throttle IS OPEN?

The noise occurs just in certain throttle position and engine speed combinations. NOT at idle, for example. But does occur when starting up from a stop, for a little bit. When engine speeds up, it seems to go away.

I DID find a hose that is on the LH side of the vehicle that seems to have cracked through. The hose is one that goes back on the driver's side, possibly to the vapor recovery canister over the LH rear wheel..

The rest of that hose has not been found yet.... However, plugging the visible part going to the throttle body has made zero difference.

The hose broke back in the body somewhere.
 
The AC clutch...... why would it seem to be engine vacuum related?
Because the little motor needs to unload itself of the AC compressor when asked to climb-over the carcass of a road-kilt frog and GMC are cheapskates as to sensors?
Sounds at least related to vacuum. Would that cause the noise when the throttle IS OPEN?

The noise occurs just in certain throttle position and engine speed combinations. NOT at idle, for example. But does occur when starting up from a stop, for a little bit. When engine speeds up, it seems to go away.

I DID find a hose that is on the LH side of the vehicle that seems to have cracked through. The hose is one that goes back on the driver's side, possibly to the vapor recovery canister over the LH rear wheel..

The rest of that hose has not been found yet.... However, plugging the visible part going to the throttle body has made zero difference.

The hose broke back in the body somewhere.

Must have gone fat-cat and added Up-hole-steery between '91 and 2000 then.

My one, there wasn't much "body" you couldn't see both sides of. Often from the same vantage pojnt! No place to hide even shoddy workmanship.
 
To check for a vacuum leak use a propane torch unlit. turn on the gas, move it over all vacuum lines if there's a leak the engine will speed up or emit a different sound. If it does you've found the leak. If the engine operation doesn't change you probably don't have a vacuum leak
 
A friend with a late 90' GMC Jimmy with a vacuum leak that no one could find. We ended up using an air tank that we rigged for vacuum. This allowed us to listen for the leak with the engine off. Without the engine noise we found the leak in a hose under the dash.

John
 
Have not had a chance to do a real search, however:

With windows closed, noise seems to be from under dash/console.

With windows open, noise seems to be at least partly outside,

When just turning throttle sector with hood open while idling, some noise is definitely from outside.

NO Check engine, so probably not in the charcoal canister emissions hoses, but I do not think that leaks there always trip the check engine, although they may set a code..
 
Have not had a chance to do a real search, however:

With windows closed, noise seems to be from under dash/console.

With windows open, noise seems to be at least partly outside,

When just turning throttle sector with hood open while idling, some noise is definitely from outside.

NO Check engine, so probably not in the charcoal canister emissions hoses, but I do not think that leaks there always trip the check engine, although they may set a code..

ISTR that the positioning of heat vs fresh air vs cooling includes a vacuum-operated swing door. "Perceived" location of source of sounds might be about right for the inside/outside klew as it could be coming up through the dash vents when fan is OFF, not masking it, yah?

Use a non-human device for detection if you can, BTW.

Human hearing has a very sophisticated "masking" capability evolved to reduce confusion and better ID close-in threats. If barriers are "in the zone" of that, it actually blanks sound in a manner similar to the "blind spot" in the retina, though NOT because the sound was not detected. Pre-brain processing of phase-angle comparison & computation, reducing potential confusion thing. See dead spots, seating adjacent to support columns, opera houses, etc. Truck cab is of similar spatial layout.

Normal hearing, for example, when not impaired by too much "civilization", has good enough phase detection to accurately pistol-shoot a rat within ten feet, total darkness. Joe or Jill average, nobody special.

Mind, the rat has also evolved, seems to know this, so moves very quietly.

One won't hear SQUAT for quite a while if you fired a pistol in any case. Best to stock a few golf balls, rather!

Humans are significantly all-around more competent, durable, and deadly creatures than we ordinarily have to prove, so long as we don't start "thinking" about these things, higher-level brain, anyway.

Catching and righting a tipped-over glass of water thing. Moving rapidly over unfamiliar ground, eyes on a moving figure and NOT the ground, etc.

Oh.. and if you track this intermittent squeak down to a starving rodent in a Chevrolet Operating Engineer's blues? Just feed him some seeds, nuts, or wheat germ and he'll stop waving his union card about and go back to his post.

Mice don't actually much care for cheese.

:)
 
Betcha that the engineers ( who have for the most part earned my distain ) are not union but considered management.
Joe

Not familiar as to whether the two we've been involved with were AFL-CIO or AMA members?

They have a penchant for Buick motorcars, upper model ranges, so they may be well-off, or THINK they are, and even vote Republican?

First one, Buick 450-something "Wildcat".Dad drags into his mechanic, was handed the dead mouse who'd stuffed the air cleaner housing with purloined fiberglass wool over the winter, and told "no charge". His Engineer had died of vacuum poisoning.

A year later, Mum's LeSabre has a similar lack-of-motation issue, her dead "Engineer" had built the nest in the air-cleaner out of shredded paper-towel roll.

Some of that lot just don't give a damn about fire safety regulations if they can save a dime or avoid carrying any sort of workload, so I'd figure that second one as at least a Management Trainee!

:D
 
No mice or rats in this case.. Unless the skin peeled partly off the rotting corpse and is flapping in the breeze at the right frequency somewhere in the intake manifold, anchored by being stuck to the side by dried body juices. Or the head has detached and air is going through the throat just right to make it squeak in a lifelike manner.

Since organic material will rot further in this climate, I would expect the tone of any such noise to change over time, but so far it has not.

Now, if I have been sufficiently disgusting with my explanation so as to put the rotting rat to bed (not under MY pillow, but possibly under yours), maybe we;ll get somewhere when it stops raining. The shed is too full of equipment to get the truck under cover for investigation, so I am waiting for a dry day..

I leave you with this question, shamelessly borrowed from elsewhere and modified to suit..... "you ever been bitten by a dead rat?".
 
I leave you with this question, shamelessly borrowed from elsewhere and modified to suit..... "you ever been bitten by a dead rat?".

Nah. The reverse. Bitten into a mess of dead squirrel, AKA "tree rat", day's bag. But yah have to cook them an insane long time for all the biologicals they carry. Snakes, too.

Got a problem wit' some of dat. Glue traps have proven serious effective. But there you are, poor honest critter just trying to make a living with what hand he was dealt by the universe at-large is now struggling, no real fault of his own but picking the wrong house to "rent".

Lenox to the rescue. 18" power-hacksaw blade off the stash for the big Kasto, rounded end "guillotine" and the suffering ends. Cheaper than .380 ammo, too. Don't ask. The .22 just wasn't handy - that's for horseflies! Wasps and Hornets aren't as tough. BB gun does the job.

Mountain folk stuff. Y'all out St Louis way? "Flatland furriners"!

Y'know the "real reason" we always put our homes in a "hollow", never atop a rise?

Gets yah a 360-degree set of hills, ridges, mountains for a backstop.

Shoot whatever needs shot in any direction, clear sight of exactly what it IS, and without messing up the neighbours' kids or cattle.

:)
 
......

Y'know the "real reason" we always put our homes in a "hollow", never atop a rise?

Gets yah a 360-degree set of hills, ridges, mountains for a backstop.

Shoot whatever needs shot in any direction, clear sight of exactly what it IS, and without messing up the neighbours' kids or cattle.

:)

Too noisy..... Arrow or knife much better.

But in any case, yes, I know there is a mess of vacuum powered stuff operating the air directing "valves". Hoses there look OK so far, and all of that stuff is working,. I still have not found the broken hose other end. No clue what it runs yet.

Propane torch best idea so far, once it quits raining.
 
Too noisy..... Arrow or knife much better.

But in any case, yes, I know there is a mess of vacuum powered stuff operating the air directing "valves". Hoses there look OK so far, and all of that stuff is working,. I still have not found the broken hose other end. No clue what it runs yet.

Propane torch best idea so far, once it quits raining.

D'you have those little two-colour plastic disks here and there in vacuum lines? One-way valves or flow controllers of some sort, AFAIK. Seen a few of those de-laminate and act like organ-reeds. Not as obvious as a hose cracked at the end.
 








 
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