I had the same issue with my driver’s side upper control arm and it was getting really loud (a bit embarrassing in parking lots). I used some Liquid Wrench, sprayed into a small puncture I made in the rubber boot, and it silenced the creak almost immediately. I had to wait a couple weeks for my appointment, and the creaking sound never returned. In the spirit of full disclosure, I told the Tesla service rep what I had done – he was wondering why it wasn’t making any noise the day or my service appointment. There are others that use grease which might be a better solution, but the Liquid Wrench route worked for me.