Dave, As you say you inspected the wippen, I would examine the hammer. Either the hammer shank screw is loose, or the glue holding the head to the shank has cracked. I've seen the latter repeatedly. The idea to swap parts from one key to another is a good approach (I would start with the hammer) If all else fails, just to be sure, look for a voicing needle tip imbedded in the hammer head. I've seen that too. Peace Jean-Jacques Granas Warsaw
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