I ran my rear pads down to under 3mm. The groaning in reverse had increased to an intolerable level, so I replaced them. Didn't even bother to clean the rotors, let alone resurface them. The new pads, Wagner SevereDuty, are silent. They took about a week, maybe 4-500 miles, to fully bed in and begin working properly. Now I can say I'm happy with them. I think the stock pads are the issue. Mine had occasionally groaned in reverse, and were always a bit noisy. Now I can only hear the fronts, which are still original. I also switched the rear brackets as mine were reversed from the factory, but that does not appear to be a factor in brake noise based on what people are posting here. I'll be putting a set on the front.