A tip that might work for noisy air conditioners:
I have a couple of wall mounted AC units that were making rattling noises, where the plastic panels were vibrating from the motor action.
Some Dynamat (acoustic mass damper in an adhesive sheet, available from auto shops) applied to the inside of the panel stopped the noise. Here's a photo. I went a bit overboard on the Dynamat as it was just sitting around unused.
Note that this goes on the inside of the front panel, so it's not a huge unsightly mess when the AC is viewed normally.
Obviously, this is not going to work for noisy ACs where the noise is from the motor.