We have about 25 systems. One of them is possessed (and by “possessed” I mean in the technical sense). When the wall unit is turned on, it intermittently emits a wailing sound like a small child got a hold of the mic and was pulling our leg – but that is not the case. It plays music when we want it to correctly, but when all inputs are turned off, it emits sounds like the underwater mating call of a sperm whale, or perhaps the amplified whimpering of a child ghost from the Victorian era who died tragically of diphtheria. I’ve done all the usual stuff, rebooted it, dusted the equalizer, tried other mics and input devices (including direct wiring). Got any ideas?