Try the standard reseating all connections especially the ones at the amp board. Possibly replace the interconnecting ribbon cables although I'm not sure they affect sound but they do cause many a tron board issues. Often times the z80 chip on the super sound board goes bad but that would probably result in no sound I would think. Also start a game and step away from the machine. Does it still have this issue. If not then the movement from you playing the game is likely physically moving something in the game and as you mentioned it may be making contact with something and making the sound.