Whenever we start our 2001 Navigator, the air pump runs a few seconds. Then the pump never cycles again while the vehicle is running. I haven't been able to dig into this yet but curious about a good place to start. When the ignition is turned off it seems something is allowing the air spring bags to bleed down a little but not enough to let them completely deflate and drop the rear end down. Matter of fact the vehicle appears to be level after sitting over night.

 

This is normal for expy's that have the air susp.

 

The air ride will "air up" when you start it cuase it "air's down" when you get out, to allow easier access in and out. Yes it does still run when you are driving you just aren't hearing it. From time to time it will come on when the truck is off to "level" itself to a satifactory position in its setup. Next time you stop and get gas, lean on the truck while filling and watch it drop.