We recently replaced our hot water heater (like yesterday recently). To save some scratch and try to nix out a few other basement appliances, we went with a hybrid hot water heater.
Specifically a Rheem 80 gal (Model number: XE80T10H45U0)
The plumber installed it, filled it with water, flipped the breaker and then.....
Normally, this wouldn't be an issue, but due to its location in a closet off the media room, the sound would have to be addressed.
The datasheet specifies 49dBa of noise, but as per usual, the appliance manufacturer doesn't specify what the overall audible spectrum noise looks like.
In this case, its a single peak at 190Hz (insert spectrum graphic).
That's low enough to rumble through the whole house. And to negatively affect marital bliss.
I immediately noticed two problems.
1) The water heater was hard mounted to the house plumbing. Hard copper pipes filled with water are not known for their noise isolation.
2) The unit was sitting directly on the concrete floor, which probably wasn't 100% level or 100% in plane with the bottom of the water heater.
Both of those problems could contribute to the noise.
I should note that the only instrumentation I have for noise spectrum measurement is my Samsung Galaxy S10e (specified for repeatability's sake). Given its proximity, I would assume the actual vibration frequency is 180Hz.
I say that because that would be the 3rd harmonic of the 60Hz (mains frequency) bits inside. I'm reasonably sure there are no variable stage or frequency drives inside (aside from the fan, which is not the source of the noise). I'm also reasonably sure that my phone's microphone isn't calibrated to within 10Hz.
I'll address the floor-water heater interface in a separate post, but for the copper pipe, I'll run off to the local $HOME_IMPROVEMENT_WAREHOUSE for some flexible couplers to replace what I've got.