Happens only on takeoff and stops around 10 mph. If I accelerate slow, its not as noticeable. If I accelerated fast, it doesn't do it normally. Could it be the trans mount or loose torque converter bolts? I Installed a t case drop (trying to get rid of it soon) and it seems to have gotten a little louder since then. It's done it for a little bit now I just haven't been able to track it down. You can also feel the noise in the shifter when it does it too. And if I'm already moving and give it gas it will not do it either. It only happens on takeoff from a stop.

 

Hard for me to tell exactly, but sure sounds like what I had when the exhaust was thumping on the cross member. Changing up the motor mounts fixed it for me, but I know the tranny mounts is probably compromised too, so that will be getting changed up here in the next couple of weeks.

I'd crawl under there and see how much room you have between the cross member and the exhaust. My guess you see that they aren't touching with the engine off, but that you won't see a lot of space between them, and with worn out mounts, everything just starts shaking.

 

X2 on this. Also, if back leaf springs are shot, it will ride low and stuff hits and rubs on other stuff. Ask me how I know...

 

I was thinking mounts probably. I'll check once I get a chance. The rear leafs were recently replaced with new ProComp packs.