did your tech at your gmc dealer tell you fan clutch?If GM knows what the problem is and it is as simple as the fan clutch (what they told me) or the belt/tensioner you would think that first the diagnosis would be consistent and that a repair would be available.
or was there an official tsb or repair bulletin from gm?
I couldn't believe how sound travels through that engine and bell housing. Using a modified stethoscope on mine, I was dead certain that the sound on mine was in the area of the egr components (they make crazy end-of-world sounds through the stethoscope anyhow).
I was wrong.
temporarily removing the aux belt, per gmc instructions posted a few entries up on this thread, proved to me that the thumping sound on my truck is belt, tensioner, or both.
fan clutch is mighty close to aux belt tensioner/aux belt on these engines
did you try/have dealer try the aux belt temporary removal test?