I am experiencing a repeated clunking/knocking noise from what sounds like the rear end of my truck when I let off the gas at around 40 mph or less. I searched and found a lot of posts on clunks when taking off or accelerating, but came up empty on my problem. Truck is an '01 F150, 4.6L, 5-spd, 3.55 LS diff, 89k miles. Anyone have any suggestions or similar experiences? -Tim

 

jack the rear up off the ground and with it in neutral take the pass side tire and go back and fourth with it
if there is alot of play between to and fro
backlash in the rear gears is the most likely cause
check the rear u-joint also and while your at it do the front one also

...zap!

 

There is about 1/4" of free play as measured at the tread of the tires when I spin one tire. I pulled off the diff cover and the ring and pinion gears look fine with a good wear pattern. The spider gears are a different story. The left and right spider gears in the carrier look fine but the other two are worn unevenly, almost down to a point on some teeth. I have owned the truck since it was new and have never been into the rear end. I haven't driven my truck hard and miles are almost all interstate, so this wear seems unusual. I guess I am due for a new LS diff...

 

Don't get excited. Spider gears always look funny. Doesn't mean they are bad. I have never replaced a set yet unless they broke. I would lean more towards a u-joint issue.

Robbie

 

Noise on deacceleration can be a diff. carrier that has side bearings that are worn, letting the carrier move from side to side. Also, a spalled outer pinion bearing and/or loose or improperly torgued pinion nut.

Any sign of lube leaks at the pinion? How does the u-joint at the pinion feel?