Ok my tuppence worth.......Guitars are very very simple peices of kit...they usually make noises for one of two reasons. 1. they are fitted with single coil pickups and are being effected by other electrical equipment (strip lights are the worst) 2. Poor earthing, which is your problem, if you think about it, you are acting as the earth when you touch the control plate. If you check the earth wire that runs from a pot to the bridge you will find it has a dry solder joint or its not contacting the bridge very well.