2001 GMC Jimmy metal grinding sound

I have a problem with my 2001 GMC Jimmy in that it will sometimes make a very loud metal grinding sound that increases with speed. It sounds like it is from the front of the vehicle. When I slow down to 10-20mph it becomes a staccato like clunks like a metal object up against bicycle tire spokes. So the faster I go the faster it gets. When I get up to about 50-70mph it will then sound like a loud whine. It also shakes the vehicle. When I slow down and come to a stop it will make a loud clunk and then stop. Sometimes is starts back when it get up to 20-40mph. Sometimes not. Some days it will occur and some days not. A mechanic is currently working on it now and says that the universal joint looks good and that the front differential was drained to look for metal fragments but that turned out OK too. So right now he says he doesn’t know what the problem is. Any ideas?

If it’s four wheel drive, I’d be wondering if maybe it’s stuck in 4hi. Remove the front driveshaft and test drive.