Alright. Under hard acceleration in 2nd gear going 10~ there is a vibration/ratteling sound under my feet. I say 2nd gear, but it does it anytime the engine is low in the rpm and is asked to work hard. If it downhifts the noise doesnt happen. This happens in a straight line and turning. If I WOT from a stop it doesnt happen.

Edit: I found a way to describe it. Think of a washer slipped over a shaft. The washer is to big for the shaft. Now spin the washer at a high rate of speed. I imagine the washer being off balance and vibrating as it spins fast.
Another way would be similar to spinning a caster (wheel) faster than it can handle and it eventually spins fast enough that it starts to vibrate. Its not spinning smooth.
(I dont know if this makes any sense...)

I have changed the rear ds u-joints.
Im just trouble shooting here, but does the front ds turn while in 2wd?
Could there be something wrong in the t-case or tranny? Trans shifts ok.

2001 5.9 4x4
Stock suspension

 

Have you checked to make sure it isn't just a heat shield or some part of the exhaust rattling around a little? On one of my cars after some exhaust work a heat shield over top of my cat was rattling at idle and low rpm, but would stop once the rpm got higher. Sounded like something more serious than the heat shield. Just get down under the truck and bang on some stuff and see if you can find where the sound is coming from. If you can't find anything that way, then you can move on to more serious stuff like a t-case.

 

i 2nd the heat shield.

 

I checked the heat shield. Its all tight. And banging around yeilded no noises.
The noise isnt just rpm related, its load and rpm related, if that makes any sense.
Someone said to check the snap ring on the rear of the t-case. Do I have to pull the shaft to do this?

 

Yeah, you do.

If its only under load...... might be worn mounts, so, under load, two metal parts that don't normally touch (I.E. when you are under there looking....) end up hitting when you torque the motor. Might look for shiny worn spots on the various metal parts down there.

 

ping maybe?have you checked your plenum for oil?pinging often sounds like a little metallic rattle under load and will go away when in torque peak or not loaded.

 

I get the same noise when I put my trans in reverse when the truck is first started. It also happens if I rev the engine in park, but it won't do it in neutral. My sound comes from the transmission.

 

Mine does it in reverse too! Just started today....