Help, where's the tin can???

This is maddening. About 3 weeks ago I first noticed what sounds like the dead ringer for what I imagine a tin can dragging off my rear suspension would sound like. I noticed it pulling up the driveway an into the garage. I look sort of silly crawling all around the car for a mystical dragging can I couldn't find. I heard it probably 2 or 3 times since so about 1 in 5 times. It happened again today as I was backing down the driveway and it sounded even louder. With my window down I'd estimate it could be heard a block away as I drove off. It disappeared within 10-15 seconds after I got up to speed.

Anybody have a clue? My gut says it's somewhere in the exhaust system but I don't know where and don't see an obvious issue. I'm heading to the dealership later this week for the rear diff. fluid change so any help more accurately describing any potentially known issue in advance of the visit is greatly appreciated.

Thanks.

I had the same noise backing out of my driveway one day in my e90. Turned out to be a small pebble in the brake rotor and the guard behind it. Sounds just like a tin can.

Like 328dude, I had exact same noise and turned out to be a small stone between brake rotor and water/dust shield

Very common problem. Has happened on every BMW I've had, dating back to my 2003 540i. Stone between the rotor and the backing plate. In short order, it will fall out on its own.