The drivers seat in my e90 has been making a clicking\popping noise when braking hard and/or turning left (I am sure same condition could exist when turning right though). After quite a few searches I read where a lot of folks described a similar issue and stealership "could not replicate" so they basically sprayed lube all over the seat under carriage and that fixed the issue for a little while but it came back. I am not a fan of the grease everything and hope that fixes method myself. My initial thought when I was driving is it was coming from the pivot between the seat bottom and seat back, which turned out not to be the case. After some investigating I found a way to reproduce the issue, find and fix it, so I hope this helps some...
Steps to reproduce and fix:
1. Adjust the drivers seat all the way forward and all the way up.
2. Adjust the head rest up so you have a decent handle to grab on to apply pressure to so that you can simulate the force/motion of your weight when braking.
3. Push and pull the seat forward by the metal head rest supports and see if you can replicate the popping/creaking noise. NOTE: You will have some play and a bit of noise due to the "play" in the back of the seat tilt adjustment, but you will know the popping creaking sound when you hear it (the one that has been driving you nuts!)
4. While doing step #3 with one hand, look under the seat and use your other hand to feel where the sound is coming from. Yes this position is as awkward and comfortable as you think it is.
5. See the image below with the red arrows. This is where my sound was coming from. My first thought when I was driving was the sound was coming from the the back of the seat as I could kind of feel it in the seat back, but that turned out not to be the case. Pic taken from front of the seat. The issue was coming from the front pivot point for the height adjustment (the one on the front corner of the seat closest to the driver's door.
6. To tighten that bolt you need to remove the seat track rail cover to access the nut to get a socket on it. To remove that cover simply lift straight up on it and it pops right off. See the grainy out of focus pic below (sorry). That is my hand attempting to show you where I grabbed it to remove it. The other pic shows the clips molded into the cover to show how it fastens on to the track. I have a habit of breaking stuff like this and like to see how these things fasten on before I remove if at all possible. The removal was actually pretty easy once I figured out how it fastened.
7. With that cover removed, you are now ready to tighten the bolt. Use a T-27 torx on the inside and a 13M socket/wrench on the outside. Mine was more than finger tight, but I probably got 1-2 turns on it to tighten.
8. Now try to retest and see if the popping noise is fixed! Also make sure your seat still adjusts up and down without issue.
9. Re-install the cover removed in step #6. This step took the longest for me I think as there is not much room to work and angle was awkward for me, but may be b/c I am a left.
10. Happy trails!