Hi,
a Fender Princeton amp came to me with its reverb making bad noises at the slightest bump or movement of the amp. Bit like an elephant walking on piles of sheet metal.
When I removed it from the back of the amp and opened it up, I saw what the problem was in a second.
see link for pic of a MOD reverb tank.
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Note the piece of foam supporting the two springs in the middle. No kidding, it was still there.
..... Phil