Noise when letting off the accelerator

I have a 2015 Kenworth t680 with an MX13 and now an 18 speed. I have owned the truck for 2 years and ever since I've owned it I get a noise when letting off the accelerator. It sounds like it's coming from under my feet when driving. It's just a very brief clunk that occurs when cresting a hill or letting off the accelerator. The noise is never consistent and it happens randomly. Sometimes I can go a day without hearing it and others it happens frequently. I always thought it was coming from the transmission but the original 13 speed tranny that was in the truck failed and I replaced it with the 18 speed and the noise still occurs. The clutch was replaced when the transmission was replaced but off the top of my head I am unsure if the flywheel was touched. I'm hoping that if the flywheel needed replacing the mechanics would have done it too when the clutch was done but you know how that goes. I'm just looking for ideas on what to check out since I'd like to not keep pushing my luck and end up on the side of the road somewhere in the future. I have an audio file of the noise but this site won't let me upload it. Thanks in advance.

 

I'm assuming they put in new U joints during the trans swap..?

Check your yoke as well, the nut can back off.

 

Engine mounts or cab mounts could be a good possibility

 

Check your cab isolators. W900’s will do that, they’re right under your feet in those, i don’t know where they are on your truck but they’re under there somewhere. W900’s do it more when you turn, but if they’re real bad they’ll bang when load on the engine changes and the frame rail moves.

 

Probably one of the rear ends. They are adjusted to have play in them (gear lash) as they wear or if improperly adjusted they will develop a clunk most often heard when letting off the accelerator abruptly or accelerating abruptly. Google Ring And Pinion adjustments for your model number on the rear end the model number will be on the metal tag on the third member The part of the rear end that is removed by pulling the drive shafts and big bolts that face to the front of the truck not the big housing that is attached to the frame. Noise like this can often transfer and sound like it is right under you when driving.

 

If you hear same noise during slow speed off angle turns it can be front shackles my 15 model was a noisy one from new .

 

Maybe could be exhaust pipe flex hitting something if it runs real close to something under cab and sound transmits to under your feet.

 

Thanks all for the responses and ideas! I'll keep you posted if I find what it is.

 

I drove a few 579 Pete's with that motor, they all had a turbo "bark" that sounded like it was right under your feet. Is that the noise you're hearing? Only happened after it was at a high boost