Initially it did sound like a leaking airbag but after reading how you are "re-setting" the compressor I'm not so sure anymore as you might have caused more damage by messing with the sensors. Have you scanned the vehicle? Before anything else, SCAN the X and see what codes come up. The cost of a scan tool will pay for itself after a couple of scans.

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BMW X5 E53, 4.8iS - 2005
Professional Stereo/Nav with DSP (modified BM54 module) - Converted to Eonon GA5166X with Reverse cam
4-Corner Air Suspension - Converted to BC Coilovers