Sounds like the transformer is on the way out. It happened to MP-1 and it got progressively worse until the unit shut down. Time for a visit to Lake Woebegone.
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It could also be the power supply cover. We've seen some of the covers resonate on older MP-1s that have had power supply updates without the transformer being updated. It does not mean that the transformer is bad, but it can cause the cover to vibrate. Easy enough to fix if that's the case.
If we update the unit to the MkIII, the power transformer gets replaced along with a host of other stuff...