If I understand correctly the noise affects all the ceiling speakers not just one. The only practical way to shield these speakers is to install them in a back can connected to a proper ground. If it were me and it came down to that I would rather rip it all out and do it the right way. Self powered speakers all over the place with power and line level audio wiring (especially home run from each speaker) is not the way to go. You're only asking for trouble.

-Hal


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