1999 Jeep XJ RWD Auto, 170000 Miles on the odo.

Recently had the rear diff rebuild, rear springs and shocks replaced, draivehaft U-Joints replaced, and had the transmission service. All suspension components and diff components were stock replacements. Large Locally owned reputable shop and actually had to tear the diff down twice due to a defective part. Assured me it was reinstalled correctly, been driving the jeep for a few weeks.

Afterwards I noticed that when slowing down, there is a rumbling noise. It seems like when there's slack in the drivetrain is when the noise is noticeable in that short moment when the engine begins to work against the drivetrain .It also seems like its very faint when slowing down in generally, but can not hear anything cruising or accelerating. I can feel the rumble under my feet and can also hear it. If i put my foot slightly on the gas I can get the noise to be more prolonged.

Ive gotten some vague reasons from sources for the noise from my rear part of the transmission going out or even the angle of the drivetrain with the new springs causing an uneven load on the drivetrain. (The new Springs do raise up the rear end quite a bit due to being new)

Please help me make some sense of this odd noise. I swear up and down that I did not notice this issue before, and they continue to tell me it is not work they have done, which I am inclined to believe this, however after spending large amounts of money on things that could be causing this issue I would like to make sure or get some opinions one way or another.

 

It doesn't sound like the differential is the cause because the noise is not happening when accelerating or coasting.

I would start by getting under the Jeep and start pulling and pushing.

Check the tension on the parking brake cable adjuster, if it is too loose, it can hang low and hit the drive shaft at different points of the acceleration and deceleration cycle, and make a hell of a rumble that you feel in the floor.

Other than that, check ujoints, and even check the engine bay, see if anything is loose, out of place.

 

I checked the driveshaft, as well as the parking brake cable. The parking brake cable is where it supposed to be and is properly secured. I did check the driveshaft and both U joints appear to be greased properly and are installed correctly. I did notice though that if I pushed up and down on the driveshaft close to the transmission the shaft that goes into the transmission does move up and down minimally, not a lot, but it seems like it does move slightly.

 

I’m going to be following this thread as mine does the same thing. Every time I let off the gas around 50mph or so I get a growling sound from what seems like the rear end. I too thought it was the ring and pinion. My XJ came with a service log since 22k miles as it was a fleet vehicle. Low and behold, the rear differential was rebuilt in 2017 which was just 15,000 miles ago as it wasn’t driven much after that...

I don’t believe it’s transmission related as my Jeep only has 130k on it. Aside from going into Drive from park, which produced a clunk (if I recall my 98 did the same thing), it has no issue going through gears smoothly and doesn’t slip while driving.

I’ll do some extra poking around on mine and see if I find anything.

Id also like to note my rear suspension appears to be new. From Crown. I didn’t put it in but the XJ does have a bit of a forward rake. It appears to be settling over the last 4 months or so, but these must have been the HD springs or something because it sits high enough to almost be a 2” lift over stock and has my rear shackles damn near vertical. Typically I’d think this would cause issues at higher speeds, aroun 65 - 70 mph, but it doesn’t. So a couple things we have in common right now that I’ve been meaning to address myself.

 

it could still be the drive shaft u-joints, they are now spinning in a different spot, since the new springs lift it a bit.
you could either put up with the noise, or try changing out the rear most u-joint to start, i will bet that is the problem.

 

Both u joints are new and were replaced at the same time.

 

How does the transmission mount look? Could the exhaust hanger or pipe be hitting under there because it is loose? Are the engine mounts good?

I would be hunting for weight shifting forward when stopping, and that would mean something is rubbing against something else. If the sound is rotational, then you know it is something that is constantly turning.

 

Crawl under your XJ and locate your muffler. Just above the muffler you’ll see the muffler heat shield (looks like a thin aluminum cookie sheet) which is supposed to be bolted to the floor of your Jeep. Grab it and check for play.

Crawled under mine this morning and noticed it was hanging by just a rear bolt, resting on the muffler. I pulled it out. Drove 30 miles to work and the growling noise when letting off the gas/decelerating is now gone. Don’t ask me why it didn't make any noise during normal operation. Only deceleration rumble.

Was one of those “so simple it’s stupid” problems.

 

I had something similar on my Ford Ranger. Whenever I put the truck in reverse, i had a somewhat faint high pitch squeal. I figured my trans was starting to go. One day I had my dad start it, put it in reverse, and hold the brake. While it made the noise, i crawled around. I found that when I touched the exhaust, it got quiet. Turns out a little adjustment (hammer), and it was fine.

Stupid stuff will get you every time.

On another note, what tires do you have on it? Sometimes when tires get some wear on them, they can start to make some noise vs when they were new

 

So I returned the car to the repair shop, and they gave an analysis.

They found that when the leaf springs were replaced, the trust angle became off (They explained it was now out of spec). Coupled with the 3-4 inches of excessive lift on the rear due to the springs being new, they believe the drive shift is binding, causing the vibration.

They are going to remove the springs and have them sent back to the manufacture for adjustment, balance the driveshaft, as well as reinstall the springs with the trust angle being correct.

This sounds like my issue but I will not find out until next week.

 

Yeah 3-4 inches of lift without correcting the angle of the pinion (the driveline input to the rear differential) can certainly cause a problem. Usually most noticeable when braking or accelerating when the axle tends to rotate down or up though. Noisy when unloaded sounds like bad u-joints or failure to grease the splines where they enter the tranny/xfer case.

 

When I put my Crowns in, although they only added a 2 inch lift I ended up with a 13 degree pinion angle and driveline noise. I got some 6 degree shims which brought it right into line (1-3 degrees) and fixed (most) of the racket.

 

Like how? Make 'em flat?

So I take it you don't want any lift.

BTW how are your front coils? If they're worn maybe a better approach would be replace them. Moog 782s are popular (although mine clack).