If it's only one side, I would bet it is one of the suspension airbags that has developed a leak. I had a leaky valve in one of the rear bags in my 01 X5, and the whole rear of the car would drop overnight. Hit the key in the am, and on would go the pump, and it would come back up. Ultimatley, it was about $500 to get the airbag replaced, and not covered under the wonderfully usueless warranty extension from Mercury Insurance. I had no display messages on my dash for this issue either, but I did get the occasional Self-Level message. One out of 5 times it was the fuse.