Check the radio antenna. Make sure it isn't resting on your cell phone ;)
Check that the outlet is wired correctly. Check the ground.
Look for inductive loads (motors) on the same side of the bus in the box. AC, E36 M3 grinder, well pump... etc. If this is the case and you can't move the load - you may need a capacitor to help balance things.
Make sure the ground bus leads to a rod sunk in the ground and not just hooked to a water pipe. If it is hooked to a pipe - follow the pipes and make sure they get to ground without encountering a piece of PVC that someone repaired it with. Then go buy a grounding rod, 10lb hammer and wire.
Failing all else, google ISOLATION TRANSFORMER