New X3 28i owner - What is this sound I hear?

I'm driving a new 2012 X3 28i. It's my first BMW and so far I'm happy.

I've read the complete owner's manual, but I haven't yet been able to figure this out...
Sometimes after parking in my garage and turning off the engine, I hear what sounds like a fan (or pump?) sound coming from the right rear wheel well (when standing outside the vehicle). It stops by itself in a minute or so. This doesn't always occur, and so far I haven't been able to correlate the sound with anything else about the vehicle.

Anybody know what this could be? I'm assuming it's normal, I just want to know what it is.

Thanks,
Dave

I have it as well.

I've heard it as well. Mine also made a beep noise and looking inside, the center console hazard button was lit up.