I had this, the sound comes from the pads moving about in the caliper. nothing sinister, my dealer (I actually know the mechanic personally who worked on my car so could have an honest and frank conversation with him) solved the problem with copper grease, no doubt it will come back, but for now it has gone and is no longer annoying me. My brakes also squeal more than a piggy being chased by a wolf, but again this is down to the fact that EV brakes don't get used as much so they wear differently, I have had the calipers stripped and cleaned, the pads keyed and the problem goes away for a couple of days then starts again, it's just one of those things I have learned to live with.