6MT Weird Transmission Grind in Reverse

In the last two days my 6MT 05 has developed a weird grind in reverse.
All other gears work fine, no grinding and no issue when putting it into gear.
Putting it into reverse is no problem, but once I start to back up I feel the grind in the accelerator and can hear it. Not super loud, but low and very present.
When I bought the truck the dealer said they had the transmission rebuilt (and I called the shop who did it and they told me that yes, they did rebuild the transmission).
I haven’t greased the U Joints in probably 7K miles, so I’ll start there.
Has anybody else had something similar happen/any advice?

 

Well I greased the U Joints and the issue has become intermittent now? It’s hit or miss. Sometimes backing up an incline (not anything crazy, just a slight slope) with the steering wheel turned a little bit I hear/feel the noise, but other times it doesn’t. So far on flat ground I haven’t heard/felt it at all.

 

Got another update. The sound is no a longer grinding sound, but more of a low growl with a chirp. The sound still only appears in reverse on a slight incline, close to the “bite” when taking the clutch out.
Is my reverse going out and that’s the problem?

 

Well it has been a little over a year, and the noise mostly disappeared for the year after greasing the zerks.
I just started hearing the noise again, and like a dummy I didn't take videos of it at its worst or before I greased the u joint zerks again. Here is a video of it being intermittent. Sound should be probably at max, and what you're looking for is a "burp" or a "chirp" about 4-5 seconds in and then the same thing about 12 seconds in.
Any ideas of what it could be? U Joint failing?
https://youtube.com/shorts/JA0OgapbEhU?feature=share


Edit: I'll keep trying to get videos recording the noise.

 

You have a bad universal joint.

 

That’s what I’ve been thinking, but there’s no excessive play in my U Joints and the noise seems to go away after greasing the zerks. Is it just “failing” but not dead yet and that’s why I haven’t seen any major issues?
Edit: also it doesn’t make noise/vibrations/etc. any other time than when in reverse, which is why I’m puzzled.

 

The fact that greasing them changes the noise signature pretty much confirms the diagnosis. In reverse, you are stressing the trunnions and caps 180* from their normal, so no real surprise that it only happens in reverse. This is NOT a sign that your u joints are ok. They are obviously not totally shot yet, but only a fool would ignore this sign and wait for them to get worse. They may work for another 20k or 40k as they get worse or they could fail catastrophicly tomorrow. Having a driveshaft come loose on the freeway is not a minor incident. With rapidly rotating parts like these, once they start to go, they tend to go downhill pretty fast.

 

You cant properly check u joins on the vehicle. remove the drive shaft and look at them while manipulating looking for excessively loose or tight spots, rust or dust coming from any of the bearing caps and excessive wear.

I'd be surprised if you didn't see these failures. And like @lr172 said turning the shaft in reverse reverses the stress in the bearings.

 

Agree with above. You have to drop the DS to properly check the joints. It's not just the cups getting rusty and hollowed out that your looking for. You need to flex the yokes around and feel for tight spots

 

Thank you all for the advice! Looks like I’ll be ordering some U Joints this week (might as well replace all of them while I’m down there).

 

Sorry last question all, should I replace the carrier bearing while I'm down there? Truck has 168k miles on it.

 

IMHO, yes. I just did this 2 weeks ago on my DCLB w/175k. THe carrier bearing was not that bad, but still the lower rubber had compressed, leaving the bearing assy sitting about 3/8-1/2" lower than where it was when new. the bearing itself was in great shape, just the rubber had aged and no longer holding the bearing where it was designed to be. Get the Spicer brand, it is the OEM for that part. Only $70 at rock auto. There are several versions, so do some research here to be sure you get the correct one. U joints were $13, so absolutely no reason not to replace all 3. find some guidance on the CB, is it has both an up down and fwd/rev orientation. The part has an arrow for the latter.

Mine showed no signs of failing, though I did lift the truck 1.5". While replacing those parts, I also reset all of the driveshaft angles to optimal. Was shocked how much I could feel the difference in how easilly it accelerated. Unsure if that was from the new u joints or the angles or both. I did have a slight shudder when accelerating from a stop and that I was sure was the angle. A sagging CB messes with the angles and why you want to replace when it sags.

 

it is worn out and can be checked to confirm.

I took my propshaft to drop off at a driveshaft shop to install IEDL poly carrier bearing and while in there pulled the trigger on new good quality U-joints, and other standard stuff like balancing of the shaft. It was not expensive. Spicer parts.

Didn’t take it to the extreme of using phone angle app to re install.
Measured what height it would normally sit at using OME lift spacers and replicated the position with sleeves I had to get for the IEDLS and I believe flipped it upside down, with flat bracket, required for DCSB.
Spacer tube sleeves were basically DOM welding supply stock that happened to be the right dimensions from a hardware store and longer bolts.

haven’t had issues with it.
I try to grease it every oil change. One of the zerks is a pain to get to.
I’m not sure if that’s unnecessarily frequent and what the real recommended miles is. Would definitely be more convenient if possible to get away with greasing every 10 or 20k etc instead of 5.

using grease gun and red N Tacky.
That one PITA zerk (of 5) usually requires taking some stuff apart, finding the right access angle, and changing from my LokNLube tip to the original tip (got a new one when it failed) to fit the tight space.