My son has a '99 Cherokee Sport. A/T w/4.0. He's hearing a knock under his feet when he starts to take off and pulls up to a a stop sign to stop. I initially thought it was an axle joint or possibly a motor mount/trans mount. He's now telling me he can hear it occasionally when he's @ idle. This being said, he hears the knock every time he takes off or slows down to come to a stop.Any ideas what it could be? He says he doesn't hear it once he's up to speed but he also has a loud teenage muffler system on it :-) Any guidance would be appreciated.

 

Check the exhaust hanger at the transmission crossmember. The pipe may be hitting it. If it is, you need a new transmission mount. And if thats worn, the motor mounts be be worn as well. It is also possible the hanger itself, where its welded to the pipe, has broke free. Another thing to check is the heat shield for the convertor. the tack welds can break and rattle like crazy, usually at low rpm. Its a resonance thing. But that tends to sound like a can full of marbles when it goes, not a knock.

 

Check the exhaust down pipe or down pre cat touchuing the oil pan.

 

Trac bar at frame mount will knock at your feet if it's toast. Grab bar and push up and down, if it's bad enough to knock you will know.

A small pry bar under those mounts will show a worn mount pretty easy, I would look at the tranny mount too.

 

Sometimes the splined shafts will clunk in the drivetrain when they need to be slightly re-greased. I'd try using a pry bar to test the ball joints and the universal joints off play.

Sometimes noises can be non-vehicular. I once had a loose golf ball that would clunk against the side of the vehicle's body on turns. I had checked the universal joints and breaks many times before I looked inside of the vehicle for moving parts.

 

Possibly try pushing and holding the transfer case lever forward. When the bushings in the linkage wear they will also rattle. I’ve had a few Jeeps where I had to do this everyone I came to a stop. They usually only did it while idling. If it’s doing I on takeoff then they are probably very worn or the trans mount is bad.