I have a 2003 suburban 2500. I noticed this about a month or so ago. it is fine when you are in city traffic but when you are on the highway or interstate following behind other cars at about a car lengths apart it will make a wierd noise. it sounds like somebody is taking a rubber mallet and hitting the roof around the sunroof. it will make this thumping sound as long as you are close to any vehicle at highway speed. it will quit making the sound if you back off to about 3 car lengths but if you are any closer than 3 car lengths it will make this thumping sound. it is fine if you are driving in city traffic. it is driving me nuts. any ideas on what it could be.

 

Check your weather stripping around the windshield if it has any or other pieces of molding around the front of the vehicle.

 

Also, if you have a Luggage Rack, get up there and pull on the Rails and Crossbar. Those may be loose and thumping.

 

Its buffeting from the "dirty" air created by the vehicle in front of you...
Do you have a bug deflector installed?
As previous posters suggested check the cross rails and weather strip
Also when on the road I see a lot of the rear fascia bumper flexing on other Tahoe's and Suburbans that that is something to consider (not sure how to resolve it thought)

 

Try a Ford Product...Glue

 

ok thanks. I checked the weatherstripping and it looked ok. didn't think to check the luggage rack so will have to check that.

 

checked the luggage rack and I think it is the cross bars. I noticed that they are locked in place but they are not tight. you can pull them up a little but they won't slide until you unlock them.

 

Crossbars can be removed !, try that !
Once you do, and IF that's the problem, you can see what is worn on them, and possibly replace a part to tighten them up.