I have a Jan 2014 Model S85 that "creaks" or "crackles" sometimes. Took a while, but I know exactly what the problem is - parking brake mounting bushing are worn.
The sound happens most often when starting from a stop. It also makes the sound when slowing down after driving steady speed for a while, but only under regen, never with the brakes.
For reference, I build crapcan endurance race cars and I'm used to diagnosing odd crap all the time. Here is what we did to figure it out:
With the car on a level surface, had someone "rock" the car forward and backward lightly using the motor, while I listened at the rear wheels for the sound. It was definitely coming from the wheel area and not the motor or axles. After repeatedly looking for loose bolts in the rear suspension, I finally had someone sit in the car and put it in tow mode while I poked around. With both the parking brake and main brakes released, the parking brake calipers were loose and can move laterally around their mounting bolts. They definitely shouldn't be moving that way and appear to have worn bushings around the mount bolts. The sound is happening when the parking brake calipers are shifting forward or backward due to changes in the rotor speed.
I've been trying to figure out how to buy replacements from Tesla. I'm hours from the closest service center and no one ever answers the phone or responds to messages.