Grinding Noise

I have a 2000 Chevy Tahoe and I hear a grinding noise (metallic) that lasts for a second or so usually between 15 and 40 mph. I took it to the dealer and they said it was the heat shield in the muffler. Midas disagrees and said it might be a bearing. Any thoughts or similar experiences?
Also, if it is the heat shield, if I replace the muffler with another one the noise would go away, right??

Heat shields are normally attached to the body and remain on the vehicle if the muffler is removed. Heat shields/other stuff can vibrate as harmonics/ frequency changes as the vehicle speed increase or decreases.

on the other hand...the muffler could have a loose baffle inside.

intermitant bearing noise? ...doubtful.

jimmymc