2008 sierra duramax . ive had an odd noise since i bought the truck. finally decided to snoop around. took serp belt off to rule out noise in the drive accessories. still there. started listening to screwdrivers. put it on the outlet line at my cp3 and i could hear my noise. it sounds like alarm clock bell or typewriter type deal. truck runs fine no smoke unless floored for just a second. it is egr deleted. why would it be making such a noise? its not crazy loud and noticable just annoying. i think it does have a light performance tune. nothing crazy