I have a 07' XLT Crew cab, 8.8 LS differential issue. It has a knocking noise when decelerating/ coasting that is consistent in speed with the driveline.
I recently replaced the spider gears due to them breaking, and ended up replacing the clutch packs, spring clip, and carrier bearings at that time. I refilled the diff with Valvoline 75W140 synthetic fluid and the Motorcraft friction modifier. Now I have the knocking sound when I let off the gas which was not happening prior to the initial repair. As soon as I touch the gas the knocking stops, but within seconds of being off the gas the knock starts.
I took the diff apart again and figured it was some setup issue or bearings, so I replaced the pinion bearings and races as well. I spent all weekend taking my time with pinion shim adjustments until I got a near perfect gear tooth pattern and the backlash is within spec at .010". I set the pinion bearing preload at 22 inch lbs. of drag (spec is 16-28 inch lbs. on new bearings), put it all back together, and test drove. Still have the knocking noise...
I'm not sure what else to try. I have not replaced the ring and pinion as they are in good shape with no chipping. I also have not replaced the axle bearings, but I don't think that's the issue since it isn't constant while accelerating.
Any ideas??? Thank you in advance for any advice...

 

Have you checked the u-joints and center support, if it has one?

 

Thanks for responding. The u-joints feel ok, and it's a 1 piece driveline so no center support. I wondered if maybe the output bearing could cause it?... I did jack the truck up and had my wife accelerate and coast to re-create the sound. It's difficult to pinpoint whether it's in the front of the driveline or the back, but it would be too coincidental that something else went bad while I was working on the diff. Is it possible that the ring and pinion are just worn out, and even with a good gear pattern maybe they are just sloppy?... The truck has 187,000 miles on it.

 

I agree with glc. Check those u-joints very carefully.

 

Thanks GLC and Roadie for the advice, I think I will take the driveline in and have it checked for balance and just replace the u-joints... I don't have much to lose at this point

 

The u-joints may not be easily replaceable without buying a special kit - some of them are staked in and are not designed to be user replaceable. Ford wants you to buy a new driveshaft.