94 XJ. - Whenever I decelerate I hear rumbling/knocking. It sounds almost like a semi truck slowing down. At first I thought it was something to do with the exhaust, but now I don't know. The sound changes tempo to match my speed. If I am going 60 and let off on the gas the knocking is fast. If I am going 5 it is s l o w. The sound will slow down to as slow as two knocks per second, which is slower than the valves at idle. It almost sounds like it is revolving. Now, if I accelerate even the slightest, the knocking goes away completely, but the moment I let off the gas, it returns. Also, if I brake hard there is no knocking sound. If I drive with the windows down it sounds as if it is coming from the back half of the truck.

Any ideas?

 

Bump. Anybody have an opinion?

 

It sounds like you exhaust. Some mufflers have fiberglass in them to give that rumble sound of back pressure. Maybe there is a piece of metal or fiberglass bouncing around some where through out your exhaust..I don't know. Maybe you need a new muffler or cat.

 

...sounds like a muffler issue

 

Maybe a u joint

 

I took it to the shop today to have it looked at. Turns out the baffling problem was something simple: the muffler and tailpipe hangers had broken. The tailpipe, which runs right next to the gas tank was banging on it.