A30+ Multiroom Volume ramps on its own plus Cracking sounds

I have 4 A30+ units installed. They all have latest firmware.
When the volumes ramp up and down on their own. As if a ghost was doing it.
They also have a crackling sound output at times.
I have replaced all 4 units and still have the problem.
I have used at least 2 of the “bad” units elsewhere and they did not show the problems.
I have connected one unit wired into the network, although all 4 show 100% wifi signal. They are located in same cabinet as the wifi router.
Strange, but true