Grundfos centrifugal pumps are so quiet you generally can only hear the wind off the motor's cooling fan. Get on Ebay (or a distributor) and just buy a new Grundfos of the proper size and be done with it...
I got a 1hp 6 stage Grundfos on a used mill, 35gpm @170psi. It was wired in reverse because the pressure and flow were too much. It isn't any louder than any good low pressure pump, except for the sound of the coolant spraying in the machine.
We just purchased in March of 2022, a Haas VF-2SS. Added the high pressure coolant option. We are a small shop (3200 sq. ft), 1 machine, that produces our own parts for our own product line. No outside machine work, no job shop. The shop is very organized and clean.
1 - The pump is a different pump than the regular coolant pump. The high pressure pump is gear driven. It is so loud, you have a headache by noon. When coolant kicks on, you can't hear the radio, the guy who does assembly is about to go insane from the noise. If my door is closed on my office and I am on the phone, it is hard to hear the phone call when the pump kicks on.
2 - We have to add a 5 gallon coolant bucket every other day, sometimes every day. The tank has to be 85% full in order for the high pressure coolant pump to work. Anything less and the pump gravitates, sucks air and then the coolant starts foaming up. Shop is closed up most of day and the air isn't foggy or misty, but assume it is being misted out.
I like the machine, but seriously considering selling it and switching brands due to the coolant pump issues.
I have seen other threads about this same discussion, but it seems like most don't understand that the standard pump is quiet and the high pressure pump is not. There is a difference in pumps. different manufacturers.
The other VF-1 we owned had standard coolant and we added coolant to it every couple of weeks, not daily.
I have the standard pump and it's loud as hell, and you're right, it has to be like 70% full or more to even work. Mine is a ST lathe, and between the pump and the spndle whine, it will drive you plum nutty unless wearing lots of hearing protection.
I have a Graymill and a China pump (same Tormach uses) on my other machines that use flood coolant and both are quiet, even dead headed. The Graymill can go all the way down to like 20% before it starts cavitating, but the china pump wants to be around 70-75% full as well.