Don't try to plug a polarized 2 prong A/C connector in backwards and don't use one of those little ground lift adapters.You are experiencing a ground loop. As stated above there are many reasons this could be happening, but defeating things related to A/C safety is not the way to correct the problem.
Everyone should carry at least one of
these with them to every gig and a couple of
these . I never leave home without them. They have saved the day (or night) several times during concerts.
Best Regards,
Dave
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Wm. David McMahan
Nat'l Product and Support Manager
Generalmusic USA