That noise sounds like a very bad exhaust leak, either the exhaust manifold gasket or at the flange where the pipe meets the bottom of the exhaust manifold or both.

 

I know it sounds like that in the video, but that's not what it is. It definitely sounds like it's coming from in the motor, it's just hard to pin point where. It also goes away, so it couldn't be an exhaust leak.

 

Not true about the exhaust leak going away in my experience - sometime, carbon will build up in the gaps and temper the sound making it sound like its gone away once the motor warms up.

 

Not to mention the manifold expands as it gets hot..... and may seal the leak.

Look for evidence around the manifold/block interface. It will appear as a black sooty stain.

 

For the exhaust leak, check cylinder 7 and &, both rear most cylinder bolts. When I pulled my motor I had a bad exhaust leak, and I saw my number 7 cyclinder exhaust manifold bolts were backing out and finger loose. And a Copley others were loose aswell, just check all of them.

Mine was loud when cold, and almost non existent when hot.

 

Hmm, now you guys have me wondering if it might be an exhaust leak instead. That was my first thought, but the fact that it went away made me think it couldn't be. It also does sound like it's coming from inside of the oil pan if you get close to it, but this truck is so loud that it's hard to really tell where any sound is coming from. Is looking for soot the best way to check it?

 

Before doing anything else, fix those exhaust leaks. That job is no fun, but you can't truly tell what's really going on until you tame those loud exhaust leaks. On the positive side, I did not hear anything that remotely sounded like a rod knock during that 7 second audio clip.