I've noticed my DC3 has a faint crackling sound when warmed up.I checked to make sure the power tubes were seated properly and when I touch the tube retainer on the power tube closest to the OT the noise stops so I attached a jumper from the retainer to ground,same thing.Next I attached the jumper to the aluminum tube protector which is screwed to the cabinet,no ground contact and the noise disappeared.What is going on? Can someone explain this?