Are the pickup covers plastic or metal? If metal, you'll need to stick them on with some wax to the top of the bobbins (if you haven't already done so), otherwise as the covers vibrate from loud sounds, them moving in the magnetic field is picked up and you can get high pitched squealing that you can't damp with your hand. If plastic, you should be OK, as I can't see any reason why they would cause microphonics.