So bringing it inside created problem. May be input is getting EMI from transformer. Does your transformer has static shield? If yes them is it open or earthed?
That top metal plate is okay until it not shorting else where. Now that metal plate is considered as part of another end of bolt. That must be suspended in air with touching any chassis/metal part.
Okay so then Anil pointed it out correctly.
1. Power Supply ripple.
2. Check grounding, earth and PS '0' need not be mixed. IN case of stereo, check for individual channels, disconnect all common wires between both channels. Then power one at a time. At any point no 2 paths of ground coming together.
More information on Amp topology/pic will help us to think.