Audi Repair: 2002 Allroad Air Suspension Issues, impending failure, suspension issues

Question
What could be causing the following symptoms?

1 - Air suspension loses air seemingly on a random basis, i.e. some mornings it is still at the same height as when parked the night before.  Other mornings it has lost all pressure and the car has completely dropped.

2 - Sometimes the compressor shuts down while trying to build pressure in the system. Other times it has no trouble at all.

3 - Compressor is much louder than in the past...most likely a sign that it is shot.

Logically in my mind if one or more of the air springs had leaks that the leaks would not come and go and they would lose air on a consistent basis.  This is not happening which makes me believe the issue is a faulty pressure sensor. This then has made the compressor work excessively leading to its noiseiness and what I assume is its impending failure.  Is there any other alternative root causes?

Answer
There seem to be quite a few problems with these systems.  The things to check are the check valves, the pressure sesnors and the bags themselves.  A problem in any of these can cause the compressor to over-work and burn out.  Noisy compressor I would say is on the way out.  Let me know what you find,  Jan