Knocking sound under Jeep

'97, 4.0, aw4, np231, 130k, ~2"-3" lift, trailmaster shocks, 265/75/15 tires

I first noticed this today, a day after I used 4wd the first time in the snow and after I filled the tank from near empty. There is a knocking under my Jeep that correlates with speed, not revs.
The noise is there:

• When t-case is in neutral.
• When in 2wd.
• When in 4wd.
• When tranny is in neutral.
• When both t-case/tranny in neutral.
• At any speed.

Oddly enough, the knocking is gone when I put moderate pressure on the brakes while rolling. I put my left foot on the brake and left on the gas and took it up to 30mph with no knocking. Braking action is smooth and normal. No vibrating through the steering wheel either braking or rolling. I haven't been able to crawl under it to check it out since I have very limited means stuck here at school and we have a bit of a blizzard outside right now. No snow or ice in the wheels.

I couldn't really tell if it was coming from the front or back. When I put it in 2wd to spin the rears I don't think I heard the noise but the magnaflow is a bit loud so I can't be for certain.

I'm thinking it's something to do with a driveshaft and tranny/t-case mounts but I figured it would be a good Holiday mystery for you to solve. I hope I gave enough clues to make it easy.

I appreciate any direction you guys can point me in. Thanks!

Check your universals on the ends of the front axles. My TJ did that last year and it was the front drivers side. I pulled both axle shafts and repalced both universals. Jack it up and get under there, turn the tire and wiggle it, if there is ANY play in there replace them and you might as well do both.

Look at the front calipers -- brake pad not retracting all the way from the disk, disk is slightly warped/or caliper not riding smoothly on guide pins . Pad bonks into disk on each wheel revolution.
My .02

RichP said:

Check your universals on the ends of the front axles. My TJ did that last year and it was the front drivers side. I pulled both axle shafts and repalced both universals. Jack it up and get under there, turn the tire and wiggle it, if there is ANY play in there replace them and you might as well do both.

Click to expand...

I wouldn't hope they'd go bad so soon. They were both replaced around April or so. I can't believe 1/4 mile in 4wd on a snow covered road would blow the u-joint already. How would brake force make them get quiet though?

tommyr said:

Look at the front calipers -- brake pad not retracting all the way from the disk, disk is slightly warped/or caliper not riding smoothly on guide pins . Pad bonks into disk on each wheel revolution.
My .02

Click to expand...

It can't be warped if it brakes so smoothly though. The brakes pads/shoes/rotors were done with the u's too.

Thanks guys, keep 'em coming.

I had a caliper that was not riding correctly on the guide pins and had a knocking type sound. Just for yucks you might try taking the calipers off and reseating the pistons, then see if the noise is still there. While you have it off see if they are wearing evenly or not too.....

This may sound too simple to be true, but when i had this problem (exactly as you have described) it was my muffler leaning on my driveshaft. I took some heavy guage wire (i dont have a welder handy) and 'pulled' the muffler/tailpipe out toward the passenger side. Its worth checking... cause its a $.10 fix if thats the problem. If the clicking is louder in right hand turns, then this is almost certainly the problem. Just my .02 cents

front drive shaft u-joint? if so spicer makes one w/ a grease fitting

it sounds like mine does the same thing. on mine it will go away after about 1/2 hour of driving after using 4wd. i think my front driveshaft really needs to be rebuilt is the problem

OK, I got up under it and yanked around on the driveshafts and there was no play in any of the u-joints in the driveshafts. I wasn't able to check the ujoints in the front axle due to lack of a jack. I still need to check out the muffler but when I was under there I figured there was a decent amount of room.

i have yet to find any sort of play in any of the u joints. but it still knocks like a mofo in 4wd. im just going from what ive been told is that one of the u joints are bad

Luckily I found this...my left front wheel was loose. I had the wheel off about a month ago to replace a washer fluid pump and torqued it down to 110 ft/lbs but it must have worked its way loose somehow. I was lucky to find that a lug was finger tight and was lucky that there was a tire shop right across the street to get an impact wrench on all of the lugs.

It would have been a lot easier if I had the jack and wrench that should have been under the rear seat, but they were gone. Anyone have a oem jack and wrench assembly they want to sell me??:laugh:

These are the symptoms of a loose left front wheel.

racinfan said:

'97, 4.0, aw4, np231, 130k, ~2"-3" lift, trailmaster shocks, 265/75/15 tires

I first noticed this today, a day after I used 4wd the first time in the snow and after I filled the tank from near empty. There is a knocking under my Jeep that correlates with speed, not revs.
The noise is there:

• When t-case is in neutral.
• When in 2wd.
• When in 4wd.
• When tranny is in neutral.
• When both t-case/tranny in neutral.
• At any speed.

Oddly enough, the knocking is gone when I put moderate pressure on the brakes while rolling. I put my left foot on the brake and left on the gas and took it up to 30mph with no knocking. Braking action is smooth and normal. No vibrating through the steering wheel either braking or rolling. I haven't been able to crawl under it to check it out since I have very limited means stuck here at school and we have a bit of a blizzard outside right now. No snow or ice in the wheels.

I couldn't really tell if it was coming from the front or back. When I put it in 2wd to spin the rears I don't think I heard the noise but the magnaflow is a bit loud so I can't be for certain.

I'm thinking it's something to do with a driveshaft and tranny/t-case mounts but I figured it would be a good Holiday mystery for you to solve. I hope I gave enough clues to make it easy.

I appreciate any direction you guys can point me in. Thanks!

Click to expand...

Thanks again for all the suggestions and help, and be sure that all of your lugs are tight!

My center exhaust hanger was cracked off from the exhaust (before the cat converter) so I drilled a hole through the hanger, used some pipe tape (that flexible metal with the holes in it) and wrapped it around the pipe, and bolted it to the hanger.

Anyway it's a more stable fix than just using wire to "tie" it out of the way.....right behind welding.

Go to a junkyard for an OEM jack.

i had the same problem and my front drive shaft was tight but i decided to remove it anyway and the noise went away go figure after i rebuilt my transfer case.