At first I thought it was a National Guard helicopter approaching. I had just climbed into my Model S, had my foot on the brake (with Creep turned on). I put the car into reverse and began to hear/feel a very low frequency sound ~50Hz. As I backed out of the garage, the sound gradually faded away. Total duration was maybe 5 seconds. It just happened again in a parking lot. Curious. I'm trying to imagine what could cause it. My best guess: the motor (while in reverse and in Creep mode) is temporarily caught in a kind of pulsing feedback loop until the brakes are fully released. Has anyone else experienced this? Not a big deal. I'm just curious. When it first happened, I quickly turned off the audio system, and the sound continued.