Normally I would not think of electronic equipment, which does have any moving parts, as being subject to vibration but.. transformers are known to hum sometimes. It could be a loose mounting screw or an isolator that is not performing well. The transformers in these amps are very large and need to serious mounting hardware so it is possible that is the issue. Once you get the cover off and do your inspection, if you don't find the issue, I would plug it in and let it sit until the noise starts and see if you can physically locate it. Obviously this will require some care on your part but if it finds the issue then you have a chance to fix it. If the transformer is humming you should be able to easily find it just by touching the external insulated portion and feeling the vibration. Pushing on them a little might cause the hum to change if it is a mounting issue. If it not the transformers then touch the heat sinks on each channel and lightly push them a little to see if the hum changes. Good luck and be careful.