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.
2004 Chevy K3500 CCLB DRW Duramax. Stock.
1992 Sea Ray 300DA. 30 plus foot Express Cruiser on a trailer. 13,500# boat and trailer and 69' of combined length rolling. Over length/width permitted load.
SOLD-95 GMC K2500 SLE 6.5L TD, Relocated PMD to bumper #9 res., Custom Lowes special induction tube, Triple gauges on the pillar EGT, Trans, Boost, High Idle Mod, TCC-Lock up Mod, Custom Back up camera, DIY Manual Wastegate Controller, 4" from down tube to 5" stacks, Vac Pump Removed,2007 wheels, WMI by Cooling Mist 8gph @3psi 11gph on demand