Clunk when backing up

[Craigswardmtb]

I have a 2015 SHO with PP package. It's hard to describe but often, maybe 40-50% of the time, I get a strange clunk when I go in reverse. I start to roll backwards and I feel the clunk. It's not really audible but rather feels like I ran over something. The first time it happened I actually got out of the car to see what I hit or ran over. All else seems fine just the strange issue in reverse. I have tried to replicate it but it'***** or miss and when it happens it doesn't seem to matter if I'm going fast or slow or turning while in reverse. I have 12k miles on the clock and this started doing it about 2-3k ago. It happens a bit more often lately. I haven't taken it to the dealer yet as it doesn't always happen. And with my luck I would get the "cannot duplicate" claim. I plan to take it but i wanted to have a general idea from the experts on here before I do so. Any ideas? I was wagering on some sort of issue with the rdu.

 

[SHOdded]

Does seem like an RDU issue, but could be driveshaft-related (yoke/u-joint) as well. Check the rear pinion seal for leakage. You are at a complete stop before shifting to R every time? Any diff whether level surface or uphill/downhill?

 

[13blacksho]

I had this problem on my '13. Happened since I bought it at 36,000 miles, finally took it to the dealer at 57,000. They couldn't get the car to replicate but we realized that mine would only do this once the fluid had time to get to operating temp.
I would shift into reverse, start moving and CLUNK! Then back to drive and clunk again. Nothing would happen before or after and sometimes was not there, sometimes it was very strong... Dealer found metal shavings in the PTU and replaced it, but the problem was exactly the same. They ordered a new RDU and replaced that... Problem is 100% gone! I have 2,000 miles after the new rear diff and not a hint of this since. My dealer called Ford and they had no records of this being an issue so maybe bring this up as a possibility if you take it in.

Not a bad deal while under warranty getting two new expensive driveline parts

 

[krewat]

The more I read on this forum, the more I realize that every little idiosyncrasy of my 2013 was the same as everyone else's.

Had the exact same thing. Started pretty early in it's life, maybe 10K miles. Didn't get worse, didn't get better, turned it back in at the end of lease at around 36K or so.

My 2016 doesn't do it. Yet.

I've had two 4.6l MN12's with the 8.8 rear, a '96 T-bird and a '97 Cougar - both of them have IRS - and hearing the axle shafts slap in the pumpkin, I chalked this all up to some clearance being at the "high" side of normal.

My 2001 F250, rear driveshaft made a slap/clunk on take-off, needed to **** the slip-joint splines with PTFE grease every 10K or so.

With my 2013 SHO, figured it was the same kind of thing.

 

[Craigswardmtb]

@SHOdded I have tried different degrees of acceleration and it doesn't appear to matter. The clunk doesn't seem any worse or better. Nor does incline decline seem to matter. But honestly I very rarely park on a hill where I would need to go into reverse and haven't gone out of my way to test it on hills. I'll try it out a bit more on an incline.
Thanks for the replies. I'm still wagering something with the rdu. I'll get it to the dealer sometime soon.