I've had two things cause the "grinding" noise when I had my GMT400 trucks. Like said before, the down pipe being one. So I ditched the stock exhaust and went with the larger down to 5" out to stacks. Remember Robyn?

The other one that drove me batcrap crazy and was hard to find, was the inspection cover to the trans had three broken bolt tabs on the mounts. It wasn't grinding but rattling and sounded like a grinding noise. Hit a bump and it would make noise. Decelerate and it would stop. Taking off from a stop, it would make noise. Took me forever to find and once I did, it all made sense.