Just purchased a 2005 duramax 3 months ago. My only modification is a 265 series tire. I live in a cold northern climate. Today it was actually warm for this time of year.. around 40 degrees Ferenheit. I noticed a vibration that seemed to be coming from the passenger side floor. I only noticed it at very slow speeds. ( coming to a stop or accelerating from a stop). The vibration only lasts for a second or two. Perhaps it would be best described as a clunk however it is not drastic or harsh... just something i have noticed that wasnt there when i drove it home off the lot. At highway speeds it is not noticable everything is nice and smooth. My two thoughts were steering wheel clunk.. which i read on a post however this happens going in a straight line. My other thought was the tranny shifting when its cold. I would greatly appreciate any input you might have regarding this matter. My last concern would be the hassle i may get over the tires. I didnt realize it was even a issue until i started searching for answers here. Thanks for all the info you provide. Its greatly appreciated.

What is you cab configuration and bed length?

crew cab short box. Actually i think i have it narrowed down a bit, I sat in it in the driveway last night as it idled . I believe what ever mechanism causing the idle to increase due to the cold is causing the noise. Once it warmed up it no longer did it. Almost sounded like a lever actuated. Very hard to describe but it is tied into the idle increase when its cold.

Do you have step bars on your truck? If so, make sure the brackets are not coming in contact with the frame where the cab mounts are. I see that a lot.