2021 WR Blue Pearl WRX STI Base
Why? this seems like the most likely culprit imo. Just because one car didn't have issues, does not mean that this one will.I had RCE yellow's on my 08 and nothing like this happen, so I'm ruling those out.
Regardless of the source, to diagnose, you could try lifting the car so the rear wheels are off the ground and then separately jack up the wheel to see if anything creaks and if anything is rubbing or making contact where it shouldn't.
This is a good place to check as well since it could make a lot of noise up the shock body.This bushing on the lateral link was the culprit:
After disassembling and applying A LOT of grease/silicone lube both around the bushing and inside the bushing, the problem went away entirely.