Originally Posted by footie
It's a small stone getting between the rear disc and the shield at the back of the rear disc. Mine suffered it a couple of times when I owned it and the only way I found of getting rid of it was to drive at about 50mph and make a few repeated emergency stops.
Try this and see if it helps.
Footie is right, I had exactly the same thing, drives you nuts as it's intermittent. At first I thought I had a cracked ceramic block in on one of the cats, really bad in a parking garage. A small stone will do it every time.
I had to have the rear wheel removed to have it fixed.
hope you had this fixed a long time ago, but worth a revisit in case anyone else get the same thing