I have a 1998 Jeep Cherokee with ~110,000 miles. Lately I’ve noticed that when cruising between 30 and 35 mph I get a significant resonating hum sound that seems like it’s coming from the back half of the vehicle but it’s difficult to tell because the sound resonates through the whole cabin.
This noise always happens between 30 and 35 mph and immediately goes away if I let my foot off the accelerator (even if my speed is still between 30 and 35). It does occur to a lesser degree between 60 and 70 (a 2x multiplier, which makes sense). The car is an automatic w/ 4 wheel drive, so I tried downshifting which had no effect on the noise. Loping along in OD @ 1800 rpm or roaring about in 2nd gear @ 3000 rpm I get the same noise at the same speed.
It seems to me like these findings rule out the engine and transmission. The fact that taking my foot off the accelerator immediately eliminates the noise seems to indicate that it’s not an imbalanced wheel or bad wheel bearing.
My thoughts on possible causes:
- Imbalanced/bent drive shaft
- Bad universal joint(s)
- Bad yolk (of the non-egg variety)
- Differential
These are really all guesses as I’m a complete amateur when it comes to auto repair and (especially) Jeep repair, which I’m finding to be as much art as science. Any gurus out there that can help?
Thanks!
Is the CEL (Check Engine Light) on?
It is not. Sorry, should’ve mentioned that.
The last time this happened to me was when I had a hole in my…RESONATOR, that second muffler just behind the main one. This was a full size Chevy Caprice with the “quiet package”, hence the resonator.
I would check out the exhaust system since this is the symptom of a typical partial exhaust sytem failure. The fact that no warning lights are on would rule out engine performance items.
You might want to check the differential fluid level. If the fluid level is low, you’ll surely take out the differential. My son did on his Cherokee due to low differential fluid.
Tester
Checked the differential fluid level today; looks good.
My last Cherokee was a 1994 from a GSA auction. The jeep had nice “rear end howl” From what my mechanic told me its caused by having a load applied to a rear axle with worn gearing in the differential. Nothing catastrophic will happen. I drove the jeep 30k miles with the howl. The only thing that will happen is the howling will get louder and louder you can not carry a conversation on the cell phone.
These vehicles are very common for carrier bearings going bad. Also have your mechanic check the axles where the wheel bearings ride. They get worn also. Good luck
I took the vehicle to a shop that a co-worker recommended as reputable. They were kind enough to put the Jeep on their lift and run it in drive while listening to the various parts of the drive-train with a stethoscope. Evidently the offending part is the transfer case.
The shop wasn’t equipped/trained/interested in rebuilding a Jeep transfer case. The best estimate they could give me was ~$1,500.
I did some research based on what identification I could find on the case (NP231J) and it turns out that the main bearings often wear out in these critters, especially if the rear end has been lifted. The previous owner had lifted the rear end on the Jeep about 2".
The previous owner (a friend of mine) found that by dropping the crossmember an inch using a simple home-made spacer we could bring the transfer case down into better alignment with the rear drive-shaft. A few hours in a friend’s shop yesterday and the noise is now gone. I’m guessing I’ll still have to have the TC rebuilt at some point since the bearing is probably worn goofy, but for now, the noise and vibration have been eliminated.
-J