Have 30K on the L. Just recently noticed loud noise, sounds like from under. Doesn't happen all the time, mostly to or from work on the Interstate. After cruising along at 75-80 for a few miles, when I begin to slow to 60-65 (RPM 1800-1900) I hear the vibration begin. It was real loud one time last week and at one point while doing 45-50 and then slowing for a stop, I even looked in the bed to see if someone had put something big and heavy in it. I checked underneath it yesterday and could not find anything loose or close enough to cause that much noise. Mods are Magnaflow, K&N and Airaid, and Superchip(set to stock). Today filled with gas and mileage jumped from 15avg to 18. Any help would be appreciated. Daily driver about 400miles/wk.

 

a loud noise, from under the truck, that vibrates, that feels like a heavy load in the bed? exhaust problem?
what exactly does it sound like?
if you want to get your mileage back to normal, increase your boost on launch

 

Wish I got that good of gas mileage with my L I'm doing good to get 13 mpg. !!

 

Ya what does it sound like? Is it high pitched like a heatsheild coming undone or is it lowpitched like a rearend going bad?