Strange Noise and Need Help Diagnosing

1998 XJ 2WD Completely Stock
AW4 for the 2WD
Chrysler 8.25 rear axle with 3.55 setup

Recently, my XJ has developed a disturbing noise. I notice it when taking off from a dead stop, or at speeds below 5mph. The best way I can think to describe this noise is when you were a kid and put a baseball card in the spokes of your bike. It is a rotating "thut" "thut" "thut" noise that inceases with speed. When you get above 5mph I can no longer distinguish the noise and when the transmission shifts to 2nd it goes away completely. I still have all my gears and the torque converter is still locking and unlocking as it should at 42mph. I've seached and obviously the transfer case rattle TSB won't apply to me. I really need some help with this one as I've already dumped a TON of money in the XJ lately and am pretty worried I may be facing a transmission failure. The noise is sometimes accompanied with a rough downshift at stopping speeds. The flluid level is correct, and was changed less than a year ago. It is a good color and does not look burned.

Any ideas?

Where does th noise come from ? Have you inspected the exhaust manifold for cracks and the exhaust system for leaks ?

So far it has been difficult to determine where the noise might be coming from since it occurs at such low speeds and then disappears. I first heard it when leaving a drive thru after getting coffee. I immediately got out and crawled under the XJ thinking something had become wrapped around the driveshaft or was tangled in the brakes. There was no obstruction of any kind. You can definitely hear it with the window down but its hard to pin point a location. There is NO vibration of any kind, just the thut thut thut noise that I described. I have not checked the exhaust or intake manifolds because I thought they were ruled out since the noise goes away when the tranny shifts. That's a great thought though and I will check that right away. Any other ideas? Does this sound like transmission failure?

Interesting update:

So yesterday when I last drove the XJ it made the noise all the way home at low speeds. I parked the Jeep and have fretted about it since. Tonight, when I went to leave for work, it did not make the noise once on the way to work. If it was the transmission wouldn' it be fairly constant? Now I'm thinking Tim might be on to something with the exhaust manifold. Please keep your suggestions or input coming. Thank you.

Transmission failure is highly unlikely, and usually makes no noise.

Cracked XJ 4.0L exhaust manifolds are extremely common, and at idle and low rpm's you can hear the individual exhaust pulses.

Ok so I've done a little more diagnostic work. There does not appear to be an exhaust manifold leak and the transmission mount seems fine. The "rotating" type noise I had heard a few times has not come back, but I have noticed a "chirping" noise coming squarely from the distributor. After lots of research on various threads here this seems to be a common problem and I'm wondering if it's the root cause of my noise issues. Could the "rotating" type noise I had heard be coming from side to side play in the distributor causing the rotor to hit the cap? I have priced a replacement distributor at $127 without the CMP. It looks to be very simple to swap the CMP to the new distributor and remove and reinstall the distributor, provided I don't move the crank.

I have one question though and maybe I'm overthinking this:

When transferring the scribe marks to the new distributor how do you get it right? All the threads I've read say people just put them next to eachother and make new marks? Is that right? Is it that non-scientific?

The rest of the install looks simple using the two scribe mark method.

So I went ahead and changed the entire distributor with a new unit. I did my best to line up the scribe marks and the install actually went really well. It took me about 30-40 minutes and I was definitely taking my time. This has solved the chirping noise under the hood.

Nothing beats a happy ending!!