Originally Posted by blue2010xlt

Since the noice goes away in 2WD, he doesn't have an vacuum leak.

Sounds like one or both of the IWEs are bad. You could jack one wheel up and turn it by hand to see if you hear/feel anything. Then repeat on the other side. Other option is to actually do a visual inspection.

Originally Posted by paintballdude05

The vacuum pump isn't just for leaks. It can help diagnose faulty check valves and solenoids. All the IWE's I've worked on had metal mesh gears along with metal on the bearing.