Strange rumble under the floorboards

[Billabob]

It's been happening for months now and I was hoping to pinpoint the source as it got louder, but no luck. It sounds like a broken bearing, sort of a creaking/crackling/rumbling sound. Only happens when accelerating up a hill or when I'm stopped at a light. The exhaust is tight and the motor mounts have recently been replaced, no effect on the sound. Is there something in the transfer case/transmission that can work itself loose? 4.0/BA10/231 combo if it matters.

[89 MJ]

Could it be the exhaust rattling?

[gogmorgo]

Did you check the front driveshaft for slop in the ujoints or CV? If you grab the middle portion of the CV you shouldn’t be able to move it around while the shaft’s installed. 
Was the trans mount changed at the same time as the motor mounts?

[JeepSchmidt O'Guinness]

In park/neutral raise rpm's to about 2,000 - 2,500. If the sound presents itself my money would be the internals of the catalytic converter. Try this on cold start and operating temperatures.

[Billabob]

9 hours ago, 89 MJ said:

Could it be the exhaust rattling?

The exhaust is tight at every hanger but it MAY be rattling where it crosses the transmission mount. I'll check that out tomorrow.

7 hours ago, gogmorgo said:

Did you check the front driveshaft for slop in the ujoints or CV? If you grab the middle portion of the CV you shouldn’t be able to move it around while the shaft’s installed. 
Was the trans mount changed at the same time as the motor mounts?

It is currently loudest at idle, when neither shaft spins. The trans mount has not been changed.. I've been avoiding it since the BA10 is getting replaced eventually and the mount is almost $200. Would this cause creaking?

4 hours ago, JeepSchmidt O'Guinness said:

In park/neutral raise rpm's to about 2,000 - 2,500. If the sound presents itself my money would be the internals of the catalytic converter. Try this on cold start and operating temperatures.

No cat on this one. It doesn't seem to be affected by rpm other than it being loud when taking off, its totally quiet once it gets going. It also isn't affected by being in/out of gear or clutch pulled in/out. Oddly enough it also seems to be more noticeable inside the car. I figure its a small noise being amplified by the sheet metal but it sounds like its coming from where the trans crossmember is.

[cruiser54]

Make sure the big @$$ bolts that go up into the aluminum adapter between the trans and transfer case haven't loosened up or fallen out.

 

 

 

 

[pizzaman09]

For giggles, check your transmission fluid level.  I had some bad sounds coming from a different car of mine recently some of which were rumbles, it turns out all my trans fluid has leaked out.