Get a 69 cent adapter that goes from three prong on laptop to only two prongs in the outlet to test the situation if you don't believe it.

I got paid to insert one of these into a church sound system.. they insisted<embarrassed>.
I unplugged the compaq laptop, put the two prong adapter on, plugged it in and they were amazed.

Not up to UL safety standards, but solved the problem...
By eliminating the ground I eliminated the ground loop. Even their problems with occasionaly picking up truckers and readio stations on the PA seem to have disappeared. Can't explain that one, but the ground prong on lappies is a widely known issue for sound guys.

I do not work here, but the benefits are still awesome
Make your sound your own!