I've added this link so you can actually hear it. It's a 5.3 motor. 83K miles.
http://www.youtube.com/watch?v=KyaDPutRZHE
It just started yesterday. The vidoe is terrible but it's the audio that should help. It sounds like it's coming from the #5 cylinder. I've pulled the valve cover to look for something obvious (rocker arm, valve spring) and everything is good and tight. It will sound like this until the engine heats up then it goes away, at least that's what happened yesterday. Frankly I'm afraid to drive it.
I can't get it into the shop until Monday so for now I grounded.
BTW the vidoe doesn't do the sound justice. It's loud but not a thudding loud like a rod knock.
Also, a couple days ago the motor got hot with a stuck thermostat. Not hot enough to go into overheat protection mode. It was in the 250 range for about 5-6 miles. Could this have caused this kind of internal problem?
Any thoughts?
Thanks. Bill

Hey, I have this same problem. I have a 1998 C/K 1500 that sounds just the same but it only does it at 1500 RPM and higher, not at idle. I just put a new K&N style air filter on it and drove it pretty hard today. I don't think that could have had anything to do with it. It just started today. It's pretty darn loud, and it never did it before.

that noise is uyp top..its "double time"..valve speed...