This is natural buzzing that occurs from distorting the amp. And I beleive its grounding that stops the buzzing ( puting your hand on the bridge/strings) to a point. Its the loudest with singcle coil pickups which are on most strats, but humbuckers won't be as bad. You could buy a noisegate that will eliminate the buzz or tone it down to a minimum (better to have it to a minimum as you loose frequencies the more you crank them.)