So I just had my engine swapped, everything is going well, and I believe that he did everything correctly, but ever since he did the swap, I have this rattle/knocking from somewhere underneath behind driver seat, only when I take off decently hard from a full stop.
I naturally thought it was probably the exhaust hitting the crossmember, but I went underneath and could not get the exhaust pipe to even hit the crossmember. I replaced the rubber part of the rear exhaust hanger just for good measure, still no change.
I tried wiggling everything possible underneath the rear end, and I cannot recreate the sound.
What else could cause this noise only for the first second or two after hitting the gas from a full stop.
Any other ideas of what to check for would be appreciated.

 

YAYYYY, I get to answer one for a change. Ladies and Gentleman, it is your...





U-Joints. I had the exact same problem, swapped in some new U's and away they went.

 

Thanks for the answer man and I hate to burst your bubble, but unfortunately I had him put in new u-joints, and it had no effect on the noise. I am speaking about the two rear driveshaft ujoints

 

Man, I'm sorry. Worked for me, wish it woulda worked for you. Good luck.

 

lol swing and a miss realmortician! i know how you feel though, i get pumped when i can finally be useful on here to haha. but sadly... this is not one of those times

If it is under hard acceleration, maybe you’re getting a little body / driveline twist that’s allowing your exhaust to reach a contact point??? You mention its momentary, if it was something like u-joints or something rotating, you think it would be constant….

 

Trans and transfer case mounts going bad and you are hearing that. Possible....

 

Get it on a lift with some in the jeep and someone under. Place gear selector in driver or 1st gear. Hold brake hard and mash on the gas a few times. The person below needs to look for any obvious movement. You will find your problem then.