It is likely the plastic surround near the hinge. I have fixed it on 3 E60s.

Buy some felt material and put a few square inches of it between the plastic panels and the seat near the hinge of the backrest.

Will try that today, I first thought it was coming from the front plastic surround on the seat bottom and i did test that by putting a micro fiber towel between the leather and the plastic. I will report back, Thanks!

Did this work? Having the same problem although for me it's the seat bottom as well...

Didn't work, I took every piece of plastic trim off the seat and it still squeaked. I had the car at the dealership for other maintenance and they looked at it and lubed the mechanism as well as the leather where it is in contact with trim pieces and squeak is gone .

I had this problem and ended up taking it in as I was still under warranty. They basically took the seat out, took it apart, tightened everything up, and it's been fine every since.

I have a 2013 5 Series with the squeaking driver seat too. It's been back 2x to service dept. First time they tried the felt "fix" but the squeak came back within a few days. Second time they replaced the foam in seat back since it apparently has some metal bars that were rubbing. It's beginning to squeak some again. The local BMW service guys are trying hard to resolve the problem but this appears to be a BMW design issue. Just FYI.

I expect this is NOT the same issue, but I mention it because I really couldn't tell where my driver seat squeak was coming from...

I had a squeak that was driving me crazy. Hard to follow the sound to the source, but ultimately I found that it was the plastic boot around the seatbelt lower section -- the attachment end between seat and the center console. It would squeak as it's plastic cover barely moved back and forth while rubbing against the leather side panel of lower seat. Holding the seatbelt buckle away from the seat eliminates the squeak--as a diagnostic. This fix too is done with a piece of felt--placed on inside of lower seatbelt arm boot.

Hope that makes sense. Disregard if not relevant.