This is happening on my same E53, 4.4i 2002 as well. I have slowly watched the rear air suspension get worse, and worse over time. Eventually I finally got an error message saying "Self Leveling Susp. Inactive". This is generally caused by the wear that occurs on the rear Pneumatic Springs. After time based on the way that they sit within the car they can begin to leak slowly, maybe more on one side than the other. I am writing all of this assuming that you have the Sport Package (fyi). Notice if you hear a noise coming from the rear of your car when it starts up/when you unlock it...you would be able to hear the car lifting it self up or attempting to lift it self up. At the end of this before I finally had my Pnuematic Air Spring replaced it was hissing out air that was not inflating into the bag and I knew that it was time to have it replaced. Have them checked out- both sides. Got the part from ECS tuning 188.00...Dealership (Barron BMW) wanted 250.00 for the part.

Good luck.