Lately I've been hearing a knocking noise coming from the back of my jeep, it's usually comes on when I drive for at least 10 min. I've noticed if I throw it into neutral it goes away for a bit. I've been told it could be my rear differential or my u joints. Has anyone else had this issue?

 

could be a number of things.. bad u joint, exhaust hitting x member or heat shield, bar pins on shocks, tailgate hitting bumper.. it doesn't really matter for this, but in the future its kinda nice to know what you drive.. helps get responses anyways..

 

Sorry it's a 1990 jeep cherokee

 

Is it a repeating noise like something rotating? Over bumps? What speed? Is your jeep lifted? Need some more info. I've had a few problems somewhat related to this but all different problems

 

It happens at any speed, sounds as tho something is hitting underneath the jeep. It knocks even when I'm at idle but I can throw it in neutral and it'll go away.

 

I believe my jeep has a 2 inch lift. I know it isn't stock

 

Grab your exhaust (when cold of course) and shake it around. See if it makes any noise.

 

Ok so I know it isn't the exhaust, has anyone had to replace a transmission mount or know the symptoms of a bad mount? I'm almost positive it's either that or my u joints. Just trying to get some more opinions before I go spending some money..

 

U joints are easy to check. Park on level ground so the parking pawl isn't holding the jeep in position. Push up and down on the drive shaft(s) and twist the driveshaft while holding the yokes one at a time and look for play. Any play is bad and can cause noise. Also check you leaf spring bushings and shackle bushings. Shock bushings can do the same as well. Twist the shocks and visually inspect them to see if they're torn or blown out. Hope that helps ya

 

And there's not any heat shields hanging down is there?

 

zfranklin beat me to it, but..

if you can move your tranny around by pushing on it then theres your answer. a bad joint make a vibration, sometimes consistently when its really bad, sometimes only at certain speeds or during takeoffs and while coasting. you can easily diagnose this while under there testing your mount. just twist/push on drive shaft. the joint should have no play in it.

 

did you check anything from the first couple responses to your question?? basically all of this stuff has been covered..