2008 Sierra 2500hd Vibration Noise

[jscan]

i have a 2500hd sierra with 1k miles, I started to hear a slight vibration/buzz noise at about 400 miles, it seems like it is coming from around the passenger side dashboard floor board area. when i start to accelerate at a slow speed it will start to make the vibration noise then it will go away after i maintain a steady speed of about 45+mph. It was somewhat intermitten but is starting to happen more frequently. It only starts after the truck is "completly" warmed up. It is almost like a slight buzzing type of a vibration, not overly loud but noticable. I looked under the truck and could not find anything loose. If i had to make a guess, i would have to say it has something to do with the exhaust? 4/14/08-now that i am trying to listen more closely, it does not make the sound when i stop and apply the brakes while in gear and step on the gas nor does it do it when i put the truck in park and apply the gas...only during acceleration.( initial torque of the truck while accelerating after a slowdown or a stop)