I'm noticing a medium to high pitched groan coming from the rear of the vehicle when decelerating from sustained highway speeds, regardless of whether I'm braking or just taking my foot off the gas. Only happens some of the time and I haven't been able to reproduce it for our mechanic. It is happening more frequently now and I'd rather not wait for it to become consistent. Taking it to Audi in a week or two. Any ideas? 45k miles, newish (less than 5k) brakes and tires.