My mechanic is stumped

Hey guys, I have a 2006 wagon. She's got a grinding noise in the rear left. It's not consistent, but mainly happens when I'm turning right, particularly if I've been driving for more than 15-20 mins. It also happens if I'm driving straight but on a road where the tilt of the road puts the left side suspension under load. It does stop if I push the clutch in, unless I'm making a hard right. The mechanic has checked the suspension, brakes and all the bushings and bearings he can think of, everything looks fine. I'm thinking diff but he assures me the diff is fine. Any ideas?