Thanks for your suggestions! They make perfect sense to me. I passed them along to the mechanic, but he still seems convinced the problem has something to do with the pump. The car still isn't giving any fault codes or warning lights. He says the onboard diagnostic they use on test drives is showing everything is normal, even when the grinding sound is occurring.

At this point I think I need to find another mechanic to do a second opinion.

I'll post back once I have more info.

EDIT 3/16/15: What a long, strange trip it's been! After I wrote the above post I took the X5 to several shops and a dealer as the ABS and air suspension problems worsened and no one was able to figure out the problem. Fortunately I live near a Dinan service center, and left the car with them for several weeks last summer...yes, the problem was so deep down the electronics rabbit hole that even Dinan had a hard time diagnosing it. Eventually they decided it was "likely" to be the air suspension control module causing the problems with both the ABS and suspension. I was hesitant to spend the $$$ to replace the control module without being certain it was the problem, so I asked them to recode the module, which they did. This fixed the problem for about six weeks, after which all the problems returned.

Dinan then installed a new control module, coded it to the car and said it was fixed- the suspension was leveling properly and there were no ABS issues. When I went to pick the car up after hours the left rear was sagging. I left it there and went back in the morning to talk to the master technician and service manager. They scanned the car and didn't get any fault codes. Ultimately we decided to recode the whole car, not just the suspension module. SUCCESS!

I'm happy to report that it's been a little over six months now and there haven't been any suspension or ABS issues. The problem was this: the suspension control unit is part of a computer area network. When the suspension control was flaking out it was sending out bad information to other control units on the network, primarily the ABS control unit. Clearing fault codes wasn't enough to entirely clear out the bad information, but completely recoding the car did clear it.