The truck is a 2006 2500HD with LBZ Duramax and 6-speed Allison, <19,000 miles. The drivetrain has been making what I would describe as a chorus of LOUD cracking, snapping, popping and ticking sounds during cold and warm starts when the ambient temperature is about 65*F and higher. It doesn't make the noise every cold / warm start but it does it on a fairly regular basis. The noise is almost a certainty during hot ambient temperature starts (80's & hotter) when the engine is cool or warm.

As the engine comes up to temperature, the noise seems to dissipate and eventually go away, most of the time. There have been several instances though that the noise took about 10 miles to dissipate - it's obvious and loud enough that people's heads turn at stop lights.

There really doesn't seem to be any rhyme or reason to the noise other than it is there at idle, both in gear and in park. The noise is still there off-idle but you reach a point where engine noise overtakes the noise as you are pedaling the accelerator. The noise is coming somewhere from the powertrain or something that is connected to it - the noise seems to emanate most from the underside of the truck and the wheel wells. I have had the hood up a couple times when it was making the noise and it doesn't seem to be a top end noise - it is much more obvious through the wheel wells and truck's underside.

I can not say there a correlation to engine speed - sometimes the engine will be idling and you hear a random "tick" thrown in about once every second. Other times you hear a "tick, tick, tick" at a much more frequent interval that seems rythmic to engine speed. Other times, it's a louder cracking / popping sound - almost like a hot exhaust - but much louder and much crisper. Shut the engine off, no more noise.

The only thing I have ever heard make a noise like this in a vehicle is a cracked flex plate, but the truck has done this almost since day 1. By the time I get the truck to the dealer when it is making the noise, the engine has warmed up enough where it's quiet. Finally, on an unrelated service call, a Technician heard "a noise" but the noise dissipated before they could find the source - then they couldn't duplicate it.

Any ideas what this noise could be? It has not proven to be a functional issue yet, but the fact that the noise is there on such a regular and frequent basis is keeping me from having my peace of mind.

Thanks!