What you're hearing is electro-magnetic interference. Moving the guitar around is altering it's relative position within the magnetic flux field. The fact that it happens to more than one guitar would suggest there may be an earth problem with your rig somewhere. ie the transformers are not benefitting from usual screening/earth connection OR the other probable culprits are electric light dimming switches, and chordless phone systems somewhere in very close proximity. Tip...... If things around you don't change, then change the things around you. Take your basic amp + guitar to a completely different environment/location and see if that makes any difference.