I talked to a tech about it. His advice was that it depends on the type. Heat pump water heater is about sound of a stove exhaust fan, with a higher pitch. No bass.
Heat pump for room heating is lower pitch. Not bassy. But there are regs in some cities about how close to property lines they can be.
Do you have a heat pump? Is it noisy?
Riffing off the earlier post about heat pumps in cold weather (lemmy.world/post/10270502), how much noise does your heat pump make?...