Last night when I parked my X5, I thought the left rear tire had a leak - I could hear a slow hissing sound, but couldn't see or feel any leak. So, I left it until morning expecting I would wake up to a low tire pressure warning. Instead, I found the vehicle leaning heavily to the left side and no tire pressure indication. I started the SAV and drove it down the driveway and back, expecting it to level itself. Instead, I received the "Level Control Malfunction" warning.

Here are my questions:

1. I could not hear the compressors running at all - does anyone know if the compressors are disabled when there are malfunctions in the system?
   
2. With the hissing sound I heard, I was thinking I must have a failed air spring, or perhaps a plumbing leak. However, I read elsewhere this situation is often caused by a failed ride height sensor on the low side. Can anyone confirm this?

In short, I'm looking for help to troubleshoot the system to identify the failure. If there is anyone out there with experience with this system, I'd really appreciate some guidance. This system is full of expensive parts - expensive enough I'm not really interested in shotgunning parts at the thing hoping something will result in a fix.

Thanks for any ideas!

Justin