I have a 2001 mods are K&N intake 5" exhaust Juice Edge programmer, built transmission. This weekend I towed my 8500lb trailer into the Ga. mountains. In the morning I started the truck and it had an odd idle surge from about 500 rom to 1500 rpm. Stepping on the gas peddal stopped the surging but then I had an odd whoosh from the exhaust. Once it was driven for a few minutes all was normal. It did it again this morning when I started it to go to work. I also noticed my towing mpg and in town mpg are down a little.

 

its your high idle kicking in for cold weather, exhaust back pressure valve making the sound. the two help the engine warm, mileage down due to winter blend diesel fuel. it alll sounds as if your truck is operating normal. no worries the truck is designed to do this

 

Yep, that's the Cold Ambient Package acting like it's supposed to. It will activate at idle when it's cold enough, and deactivates when you tape the brake. If you drive for a while, park, and leave it idling, it will activate again after some time if it's still cold out. The colder it is, the higher the idle raises. You should hear the shriek out of the exhaust when it's straight-piped.