I replaced the thermal paste in a RX570 I just got, used. I’ve done this a few times before on other GPUs. However, there is horrendous coil whine after this exercise. It sounds like a strip of paper caught in a fan. I read in an old reddit post that mounting pressure can cause this. I tried loosening, I tried tightening the screws. Nothing helps. The thermal pads in a couple of the capacitor (or are they inductors?) caps are a bit torn though, didn’t have thermal pads on hand to rectify. Could this be a factor?
I always replace fans when I change thermal paste, every three years if the card is not giving thermal or noise problems.
I would suggest to try and replace the fans, it’s not expensive neither difficult and could solve the problem you’re facing.