My 2006 R52 (automatic) has been running pretty well, but I have heard a rubbing noise a couple of times at low speed when turning. I assumed something was going on like the wheel well plastic had come loose, but I couldn't see anything.
But yesterday it happened again while I was backing up. So I put on the brakes. While the brakes were on, and the car was not moving, it kept happening! It was a cyclical deep rubbing noise about once every 2 or 3 seconds, lasting for a second. It stopped when I started driving forward. I can't figure out where it is coming from. It's obvious that it's not a bearing, brake or wheel issue if it's happening while the car is sitting still.

Is it possibly something like a motor mount? I did have the transmission replaced a few months ago (brand new). Is there a rubber mount there that could be rubbing?

Edit: Problem solved. It was not a mount or anything rubbing at all. It was the noise the cooling system would make when you are low on coolant and the engine is getting too hot - not the high speed fan, but some sort of steam release or something. Problem solved with fixing my fan.