I also have air suspension on the rear of my 2006 Escalade. I've never heard the compressor and i recently had loaded the car heavier and had the feeling that it was hanging a bit. So the air suspension seemt not start. I saw on the right rear damper that the rubber is slightly damaged on one side. I don't know if that's just an outer shell or if that's the air bellow itself.
I know the air suspension from my Mercedes S-Class, but the system is fully air, so when the compressor did not start, the car lies on the floor.
Is the air suspension here, just a support? Or would the back sink down, when it not works?
Could it be that my predecessor shut down the system and I still don't get an error message? Is there a fuse somewhere? How can I test whether the system starts? Where is the compressor? At the back somewhere on the axle?