Looking for Troubleshooting Ideas

[headejm]

Just finished my 4-year Challenger restoration project and took it on a short test drive. Everything pretty much went smoothly except for some final tuning that is needed. Here's my one big problem:

Backing out of my drive way I heard a "clunking noise" about every one foot going back. Didn't hear anything on the short test drive when driving forward. I jacked the car up and looked for anything obvious and everything looks okay. I checked the driveshaft and it was tight and not sloppy. I checked axle end play and reset it to factory specs. "Clunking" noise is still present. Sounds a little like the lug nuts are barely holding the wheels on but they are all tight.

Any ideas?

[torredcuda]

U-joints new?

[Brads70]

Sure grip additive? 

[1 Wild R/T]

When you put the trans in reverse it torques the rear axle nose down, when going forward it torques the axle nose up.. When the axle torques nose down the driveshaft gets closer to the mufflers, if the balance weight at the rear of the driveshaft touches the muffler.........  Just a possibility...

[headejm]

U-joints new?

Yes.

[headejm]

Sure grip additive? 

No, was told I didn't need it. I do have a bottle of the Ford friction modifier that I could add. @Brads70 Curious what you are thinking about the additive.

[headejm]

One additional comment: My steering wheel was turned to the right slightly while backing up. Turned enough to get out of a third garage stall without going in the yard.

[headejm]

When you put the trans in reverse it torques the rear axle nose down, when going forward it torques the axle nose up.. When the axle torques nose down the driveshaft gets closer to the mufflers, if the balance weight at the rear of the driveshaft touches the muffler.........  Just a possibility...

No clearance problem with the mufflers.

[HP_Cuda]

Incorrect lash set on the chunk.

[headejm]

Incorrect lash set on the chunk.

@HP_Cuda I was wondering about that. I didn't work on the differential but I have worked on a few in the past. Is there a "easy" way to check the lash and correct? I have a Service Manual if that is a good source of information.

[HP_Cuda]

Here is a good overview:

https://www.hotrod.com/articles/mopp-0308-mopar-rear-end-gears-set-up/

Good video to watch:
https://www.youtube.com/watch?v=w2kneEjXw6E

Another good set of vids (three part series which covers it all):
https://www.youtube.com/watch?v=zT77TMcZcaU

[Brads70]

Sure grip additive? 

No, was told I didn't need it. I do have a bottle of the Ford friction modifier that I could add. @Brads70 Curious what you are thinking about the additive.

I forgot to add it once in a 9" Ford and it sounded really bad when I drove it. Really bad!  Added it and it was fine. I have no idea if your issue is the same,  just the first thought that came to me when I read your post ?

[headejm]

@Brads70 If there was a prize involved, you would win it! 

Talked to a "seasoned" mechanic and he thought it might be a sticky cone in the differential. Put a bottle of Ford friction modifier in and the clunking noise went away. Glad I didn't have to remove the differential!

I will say that the friction modifier smells about as bad as anything you will put into your car. My garage smelled bad for several days. 

[Brads70]

@Brads70 If there was a prize involved, you would win it! 

Talked to a "seasoned" mechanic and he thought it might be a sticky cone in the differential. Put a bottle of Ford friction modifier in and the clunking noise went away. Glad I didn't have to remove the differential!

I will say that the friction modifier smells about as bad as anything you will put into your car. My garage smelled bad for several days. 

Good to hear you figured it out without spending a pile of money. Ya the irony is not lost that something that smells that bad goes in a "Rear end" huh!    Don't slop any on your hands or clothes !