hi everyone. i have a 2004 dodge ram 1500 5.7L hemi. i have a vibration noise in the rear of my truck, and only does it at 2000 rpms when accelerating and goes away about 2500 rpms, it sounds like a hallow vibration like aluminum vibrating. ive lifted the truck up on jacks and ran it at 2000 rpms and it didnt make any sounds it only does it while you are moving. if anyone has any idea please let me know. i also tried taking off the heat shields to see if that was making the noise but its still there with them off. please help

 

Did you look over ALL of your exhaust tubing and make sure it's not contacting anything? Particularly around the rear axle and rear shock?
rubber mount on transmission worn down? Possibly a U-joint?

 

how would you tell if your ujoints were bad. they are kind of rusty. but it only does it when i accelerate and while it has a load on it. like if im actually driving it.

 

You grab ahold of each side and twist. There should be zero play. Any slight movement or clunking and they are bad. U-joints make noise at vairous speeds at first and then get more consistent as time goes on, and yeah, they would do it either while accelerating or decel when there is a load on them.
The only otherthing that would make noise while accelerating beside's your transmission would be your rear differential. When is the last time the fluid was replaced? Due 30kmi or sooner and it takes Synthetic.

 

at about 40000 miles ago, what kind of oil do i put in it

 

if its something in the rear diff. how would i check it.

 

You can't if you don't know what to look for.
I would just change the fluid as it's due anways and see if the noise stops.
Rear diff takes 2.25qts of 75w-140 Synthetic
If you have limited slip, add only 2qts of oil and a small tube of friction modifier.
You can get away with 80w90 as it's much cheaper than 75w-140 but ONLY if you don't use your truck to tow or haul anything in the bed. If it's a grocery getter, 80w90 will suffice. I'd prefer 75w-140 in all cases myself as it provides more protection. Oil level is 1/2" below fill hole.