I have the 2 axle suspension on my 2006 4.4i. I am having a odd problem.

1. Front end will lower to bottom end equally(no air in the front shocks.)
2. This can be in a couple of hours or may not occur for a couple of days.
3. It will lower evenly not one end or the other.
4. Rear stays in position.
5. When I activate it will raise front end with the reservoir or compressor to the correct position.
6. No faults or error message.
7. it basically operates as it should except for the front end lowering while parked.

I have the ista + and I have checked the following:

1. Raise and lower each shock. Works.
2. Check pressure, and tank for leaks works and correct.
3. Check level sensors , they work.
4. I checked every thing I could via ista+
5. I Removed cover and checked solenoid for leaks , none.
6. Check shocks and airlines for leaks , i could not find any.
7. I checked calibration and it is in range.

I thought if i had a leak in the shocks that one would lower, not both at the same rate. Both shocks were replaced by BMW with oem, 30,000 miles ago.
My x5 now has 83000 miles

I am stumped, anybody got any ideas??