Same thing with mine. You will notice the leaks by lowering your car to the lowest level and then spraying something like windex which bubbles onto the air bags. At the lower setting the lower portion of the bag rolls back upwards against the shock and you will start to see some small cracks from the rubber drying out. When up raise the suspension it rolls back out and I am guessing these small cracks seal themselves as even after spraying the bags in the upper position I can't see a single leak! I suppose that the leaks are so small that at some point the bags loose enough volume that they need to be refilled hence the compressor turning on. The clunk noise stops when you "pump" some more air into the bag by setting up the system on the higher setting then lowering it back to the middle position. Apparently the actual replacement of the bags is fairly easy to mechanically inclined folks albeit they are pretty pricey.

I have to tackle the DS front one soon. I hate to see the car leaning.

I hope this helps.

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2006 X5 4.8is Titanium Silver / Black (36K miles)
Dinan Cold Air Intake, Software, Exhaust
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1996 M3 Sedan Artic Silver / Dove Grey (65K miles)
Dinan Stage III Super Charger, Software, Throttle Body
Short springs on Koni Gas Adjustable
Borla Exhaust