I know it's hard to diagnose a noise over the internet, but this one has me baffled.

96 4.0 4wd with the AX-15 manual transmission. 3 in lift and hack n' tap if that's relevant.

Everything is in good condition, well maintained. This week I first noticed a click click click when letting off the gas, but before slowing. It is definitely related to the speed of the vehicle, not the engine speed. It's in the neutral acceleration time without a load either speeding up or slowing down, like when shifting or coasting. With a load, everything is normal. It doesn't seem to be coming from any corner of the Jeep, just kind of in the center, but it is hard to tell while driving. I first thought either something is hitting the driveshaft, or hitting a tire. As the week progressed, the noise becomes more regular and louder, now a loud tat-tat-tat and you can feel the vibration in the tunnel. I thought it must be a u-joint, and I assumed in the rear since it's happening in 2wd. I pulled the rear driveshaft and drove around in 4-hi, problem persists. I reinstalled the rear shaft and pulled the front shaft. Problem persists. Now I'm really confused.

The only components left to isolate are something in the front or rear axle, the transfer case, and the transmission. There does not seem to be excessive play in transfer case chain. I'm not really sure what to do next.

Any advice welcome.

Thanks!

 

I made a video of the noise, which appears to be getting worse, so I will be parking the Jeep until I get it sorted.

I had been suspecting the transfer case, but as I don't THINK I have a stretched chain, and it spends very little time in anything other than 2-hi, I'm shifting my suspicions. Today I stuck my head out of the window while driving (woof woof) and I pretty clearly heard the noise coming from behind me. This is with the rear shaft back in. I still have the front off. This leads me to suspect the rear axle. Axle was rebuilt by me 2 years, 15k miles ago. It is a late 8.25 axle with 3.55 gears from an automatic xj. I put in a Powertrax Grip Pro (like a tru-trac) It has had zero issues previously. I guess it is time to tear into it...

XJ between throttle clunk

 

Embarrassingly, it was loose lug nuts... I hope this admission helps someone someday.

Check your nuts!