GMC Repair: buzzing noise at under 10 mph from start up, silverado 2500hd, disapear

Question
hi, i just got a 2004 silverado 2500hd 4x4 6l. vortex, my wife notice a litte buzzing noise  when start up in trafic, the noise whill disapear when gearbox shift in second gear or when the truck goes over 10 mph, i put first gear and drove it at 8 mph aprox and you can hear the noise for long as you want

Answer
Hi
I would HIGHLY suspect the exhaust system, in particular the "clamps" or "Straps" that hold the heat shield on the muffler.
Now this is my first guess.
Can you pinpoint the area, eg.. front, rear, directly under cab... can you or assistant stand outside and hear it as truck is driven by?
Can you "Power Brake" truck and hear noise? (Truck in drive, foot on brake and push on gas)
Please follow post back.....
Thanks
Pawl