It sounds like normal typewriter tick that the duramax makes. It sounds like broken rocker arm or valve thats out of adjustment. But its reall the oil film on the rod bearings, it creates a slight harmonic that travels through the block and exhaust and is heard as an audible metal on metal tick, clang, bang, knock etc.

Some things make it more noticable.

THe Edge on our 02 made it more noticable. But that thing is gone, way too many issues with the Edge. Gave up and I just ended up tuning it.

 

edge programmers screw with the timing bigtime and you get some loud rattle noises ive unplug them before and the customer would tell me the famous well you know i remember the problem didn't start until i put the tuner on!