It's a 2001 non-sport 3.0 X5 so I don't have air suspension. I listened closer this afternoon, and indeed, the noise is coming from the front, near the engine. It's barely audible and is coming from the lower part of the engine compartment; when I open the hood, I can't hear it at all, therefore I have to get on all fours and listen underneath the vehicle in order to even hear it. It still sounds like a deep metallic expansion pop. What else could it be?