Disklavier

I have a new DC1A with an intermittent problem:  There is 
occasionally a hum that comes from the amplifier - perhaps the power 
supply.  The noise does not come through the speakers but comes 
directly from the amplifier box.  Has anyone else had this problem?  
Unfortunately, the problem disappeared when my tech arrived, but of 
course, reappeared after he left.  Any solutions?

Thanks.

J. Young