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.
I have also experienced this and I agree with your analysis, i.e., creep and brakes kind of oscillate back and forth.
Is it warm there for the first time since you got the car? The sound you describe sounds like the AC compressor when it first starts up. If you hear it again you can confirm by quickly turning off the climate control and seeing if the sound goes away.
Should be easy enough to confirm. Try it with Creep off and see if the sound goes away. (I used Creep for a while, and never noticed this).