Disklavier

Thank you for getting back to me Carol
I put the credit card towards the mid to top range of keys. Under the keys are a couple of credit-card sized circuit-boards (with various chips and resistors soldered into them). What I manage to do with the credit card is simple make contact with the circuit-board under the keys and this stops the buzzing noise - even once the credit card is removed - but only temporarily. It always returns.
After stripping down the keys I found that each of the circuit boards is screwed down by four screws - which can of course be tightened. My problem is that I don't want to over-tighten these and end up destroying the circuit board. I'm also not sure whether there is more to it than this?
I'm surprised no one else has encountered this issue. It is the only problem I have with the piano which is otherwise great. 
ThanksKevin