mine did this, on my car it appeared to be the leather rubbing against the plastics on the outside of the seat where the controls for moving it are, I tried all-sorts of lubes to fix it but most were temporary, what finally worked (and this sounds a bit odd lol) took a piece of thick cardboard about 15cm x 3cm, wrapped it up in black gaffer tape and slotted it in where the plastics meet the leather, you do have to leave a little bit exposed which is why i used the black gaffer but its barely noticable and the seats have been silent ever since
I’ll try some cardboard first but I’m sure it’s coming from inside the backrest towards the base. Going to take the seat out and have a good look and I’m sure I had to do similar on my old E46 with ptfe spray at the hinge to stop squeaking.
Any bolts or clips I should be aware to remove the back?