Here's the noise:
This noise just started happening a few weeks ago and is present only when starting a cold engine. It's always louder at first, then gets quieter, then eventually goes away after a few minutes.

Really appreciate your guys' insight as to what is causing this.

Thanks in advance!

 

What's your oil pressure on start up? Sounds like valve train, ie bad rocker or pushrod but its hard to tell. It initially sounded like a leaking exhaust manifold too.

Tim

 

Check exhaust manifolds for broken bolts.

 

I have one missing bolt head on the the drivers side rear. Think it's an exhaust leak. It goes away once the engine warms up.

 

 

I would drain oil and cut open oil filter and verify there's no metal particles. hopefully it's just broken exhaust manifold bolts. any codes? what's the oil pressure?

 

I wouldn't be concerned with the 6.0L Vortec Engine until you reach 300k miles.

 

Exhaust leaks can make many sounds depending upon where they come from. Cracked manifold, loose manifold (bolt broken/missing), loose collector flange, failed exhaust gasket, etc....

I had one on an old Chevy truck that made the strangest clicking sound at the back of the engine, .............. was just a loose header bolt.

 

Thanks Hummerz! Got the parts and will install shortly.

 

Make sure engine has cooled down. Also check for any other broken bolts(passenger side too) while your in there. If it's just the rear, clamp will take care of it. Post update when you get the clamp in place. Btw, how's that transmission performing?