2010 Sierra 1500 5.3 6 speed trans Can anyone tell me what this sound is coming from

[Andrew Rogers]

[davester]

Could be anything.  I would suggest getting a mechanics stethoscope (they are < $20), to narrow down where the problem is (carefully, as you dont' want to touch the end on/into spinning bits).

 

If it requires the truck to be moving, perhaps put the rear end up on jackstands, and use the stethoscope.