What kind of professional to call for help with structure-borne noise issues from HVAC
As an engineer having dealt with window and wall mount A/C noise problems for over a half century, I can offer you the most likely cause of the noise transmission and the solution to solve the noise problem. But first, we must understand that most, if not all, resonate noise vibrations in such units are being emitted by the compressor. Yes, the electric motor (60 cycle) driven compressor, transmits shock waves from the piston or scroll device and delivers the waves to the foot mounts at the base of the compressor body (housing) and then into the structure of the entire window or wall mount unit itself. Once one accepts the premise of this conclusion, it will make it easier to resolve the cause of the vibration problem. Resolving vibration issues in almost all cases that I have encountered, starts with relieving the stress of the compressor foot mounts. As you know the mechanical vibrations created by a compressor operation are transmitted directly to the base of the compressor. And while the entire electric motor and compressor are internally suspended with a spring mechanism, sometimes the springs do not absorb enough shock waves, which results in the motor/compressor vibration being transmitted into the frame of the the A/C unit. So with all that said, the solution is... 1- Further loosen the compressor support nuts/bolts that hold the compressor in place. 2- Remove the compressor support nuts/bolts, lift the compressor (an inch or so) and install compressor mount springs to support the compressor externally, (as well as it is supported internally) which will provide the necessary removal of vibration into the A/C housing. At the end of the day, it's all about controlling compressor vibration transmission into the unit housing.