Previous users have complained about loud noise/feedback in one of the QC20 earbuds. The "Solution" was to contact Support. I did and, of course, they offered me a slight discount on a brand new pair - Nice, but that's still $200.
I played around with mine and found that the case had split just a tiny bit - allowing the speakers to feedback to the microphone. I filled in the gap with a little epoxy and - fixed!
So check that out. Doesn't take much of a gap (and therefore not much epoxy or white glue, or whatever)