I have a keyboard synthesizer which emits a 60Hz hum due to mechanical vibration of its PSU. The vibration is magnified by the metal chassis of the synth which acts as a soundboard. It’s very annoying in a quiet home studio. The manufacturer does not consider this a problem and is unwilling to help.
What can I do to dampen this vibration? I was thinking of installing some rubber washers between the PSU and the posts it sits on, but I was concerned about blocking paths to ground (labeled M1, M2, M3, M4 in the picture below).