I’ve got 2 separate Sonos systems since the S2 app won’t work with my Play 5 and Connect Amp. 

My new system consists of an Arc, Sub, Amp and 2 in-ceiling Polk Audio speakers. This is the one I’m having issues with  

When I‘m not using the system, my left rear speaker makes a crackling noise at random times. Approximately every 20 minutes. The sound is quiet, so it is not always noticeable with ambient noise going on, but when the house is quiet it is annoying. 

Any ideas what this could be and how to stop it?