Replaced motor and tranny mounts...new rumbling noise

[Fred85]

I've searched but couldn't find anyone with quite this issue. I just replaced my motor mounts and transmission mount, which definitely cured some 'thunking' noises underneath. But, now as I'm driving at highway speeds (50+ mph) I have a loud rumbling noise which I can also feel under my feet. If I lift my foot off the accelerator, the noise completely goes away; press the accelerator, and the noise/rumble returns. The more I press the accelerator, the louder the rumble, so it seems it is more RPM-related than speed-related. It also only occurs in the 50-65 mph range. Occasionally, if I'm driving and lift off the accelerator quickly, I can feel a rumbling vibration beneath my feet.

Suggestions on where to start looking? I checked the front diff pinion and front t-case pinion and both felt tight with no side-to-side play. Thanks.

'01, 197k, 4x4, AW4, 2" OME lift, hack-n-tap SYE (rear pinion is angled correctly).

 

[Dragonslayer]

Check the clearance between the tips of mechanical cooling fan, and the fan shroud.
Be sure that the fan clears completely around the shroud opening (= 360º around).

If the clearance is too small, when accelerating, the engine will lean over a bit, and a
fan tip will get caught on the fan shroud, and the fan clutch will rumble and vibrate
and beg for mercy.

Lift off of the accelerator and the engine leans away - and the rumble goes away.

 

[old_man]

Also check the exhaust where it passes over the cross member for clearance.

 

[Ben824]

Sounds like u-joints to me. If the old trans mount was old and had been squished over time then the new one should have been called since it was new and had not weight on it. I swapped mine a while back and there was about 3/4" height difference between the two sitting side by side on the table. That increase in mount height increases the angle of the driveshaft and may be exaggerating the condition of a bad u-joint. I also had a bad u-joint that left a rumbling vibration when I was on the gas. As soon as I let off it went away.

 

[Fred85]

It's not the fan or the exhaust. I realized the front driveshaft (stock btw) had a tiny bit of slop in the splines so I pulled it. It helped slightly.

Could front axle u-joints or wheel bearings be the culprit? I've definitely been getting the angry sparrows noise from the front, so those u-joints are getting replaced next weekend. I can't see how axle u-joints would cause this type of vibe/rumble noise, but maybe?

I appreciate any other suggestions.

 

[blondejoncherokee]

Rear ds u joints are probably bad

 

[Fred85]

I guess I had ruled the rear shaft out. It's a 4-year-old Tom Woods unit...no play in any of the joints. I'll need to wait until I buy a new shaft for the front to test it out though...

I rotated the tires too; no change. Keep the suggestions coming!

 

[md21722]

Front axle u joints & wheel bearings wouldn't suddenly start making rumbling noises after transmission and engine mounts. You can put your front shaft in the rear and see if it goes away. You might check for smoothness in the double cardan while it is out. TW driveshafts are not perfect, centering balls, H's, and balance issues can develop.

 

[trippled]

What kind of motor mounts did you use? I noticed a significant difference in the way my engine sounded after switching to poly mounts

 

[Fred85]

They're Crown brand OEM-style rubber mounts.

One thing I've never really questioned is the hack-n-tap SYE. Everything is tight and has been so for 8+ years, but can a hack-n-tap cause vibration issues? I will admit that the cut on the output shaft was not perfectly square, but I always assumed that didn't really matter with a hack-n-tap...

 

[md21722]

Replacing sagging motor mounts changed the driveline geometry. Its similar to replacing sagging rear springs. The u-joints get used to where they were at and now its different... its worth a shot.

 

[trippled]

Yup. They'll develop wear patterns on them. Best way is pull the shaft and move all the joints by hand. They should be easy to move and smooth. If stiff or rough they need to be replaced

 

[Ben824]

I bet dollars to donuts its the rear shaft u-joint. If you are running a CV rear shaft, you should be able to swap the front to the rear to rule out the rear shaft being the problem. I wouldn't think it is the front one since no load is on that shaft in 2wd, its simply just spinning. The rear is under load so that is why I think your problem is there since it goes away when you let off the accelerator.

 

[Miller88]

My new motor mounts changed the rear driveline geometry. It did help with the last of the vibration issues I was getting, though.

 

[Fred85]

It was totally the CV on the rear shaft. I had wrongfully assumed that because I couldn't move it underneath the jeep, that everything was good. I took it off and realized it wasn't far from grenading itself. The u-joint on the t-case side had failed (from lack of grease I'm assuming) to the point that the needle bearings were gone and the joint was just flopping around. This of course had caused the centering ball to fail as well, which in turn caused some wear on the H-yoke.

I boxed it up and sent it back to Tom Woods to be rebuilt...with sealed u-joints this time. As far as I'm concerned, greasable u-joints can suck it. They should last longer than 4 years on a road vehicle. I still have the original joints in the original stock front shaft and it is still going strong after 200k.

I also tracked down another rumbling noise to a bad water pump bearing, so yeah, it's been a fun few weeks.

 

[blondejoncherokee]

Thats what I said the rear shaft. You have to pull thenshaft to check it . You cant tell by wiggling it on the rig. Unless its really realy bad

 

[old_man]

It was totally the CV on the rear shaft. I had wrongfully assumed that because I couldn't move it underneath the jeep, that everything was good. I took it off and realized it wasn't far from grenading itself. The u-joint on the t-case side had failed (from lack of grease I'm assuming) to the point that the needle bearings were gone and the joint was just flopping around. This of course had caused the centering ball to fail as well, which in turn caused some wear on the H-yoke.

I boxed it up and sent it back to Tom Woods to be rebuilt...with sealed u-joints this time. As far as I'm concerned, greasable u-joints can suck it. They should last longer than 4 years on a road vehicle. I still have the original joints in the original stock front shaft and it is still going strong after 200k.

I also tracked down another rumbling noise to a bad water pump bearing, so yeah, it's been a fun few weeks.

Wait until you are my age and you don't have to worry about those annoying sounds, you can't hear them any more.:anon:

 

[jordan96xj]

Glad you found it.

Even if the u-joints were brand new, they probably would have taken some time to adjust to the new angles. When I did my leaf springs recently, I had just rebuilt the drive shafts a few weeks ealier, and for the first few days on the springs, there was definitely some rumble from the drive shafts (at highway speed, and while accelerating/climbing). It settled out after about 200 miles.