Noise in 9" rear end?

[Scott McKelvey]

So I've had this truck for almost 1.75 years. I drove it home 800 miles from TN to MD. I drive it daily in urban/suburban/hiway (I've put 8000 miles on it). It has the big bearing 9", which I know has currie shafts and explorer disk brakes, and 4.56 gears, but if I had to guess it has a whole Currie axle that a previous owner installed (the whole truck has kind of a 90's makeover vibe to it, and none of it feels like it was someone that would have been installing their own gears, etc.).

I've checked the fluid many times, it's full (no idea what it is, or how old, i guess). I've not changed it. And as far as I can tell this axle has been in the truck as-is for probably 20-30 years.

Anyways, if it wasn't clear, i basically have zero issues (keep in mind everything I said above). But this occasional behavior has gotten me curious enough to ask. It very much sounds/feels like something grabbing, then letting go. I never notice anything at all, except the following (in heirarchical order).

1. ONLY when the front wheels are turned to the passenger side (not when wheels are turned to the drivers side).
2. ONLY when it's warm I've been driving around (so things are nice and warmed up, and the ambient temp is high)
3. ESSENTIALLY only hear it in reverse when slowly backing up.
4. OCCASIONALLY in forward, but only when kind of drifting, as soon as i give it any gas there's no noise, which is basically why in general i only hear it so often and don't pay it too much mind.

So:

1. I sort of assumed it was a TruTrac/Torsen style diff, but those are supposed to be silent, right? No friction modifier needed?
2. Maybe I have a friction style limited slip w/ big clutch plates that needs a friction modifier? Is that a thing? Whenever i look at a pictures of 9" diffs, they always look different than when i think of a traditional diff for say a Dana 30 or 44. So i should just add friction modifier?

reverse
forward (but not really a thing, b/c I'm on the gas pedal)

 

[DirtDonk]

Sounds exactly like a tight clutch type limited-slip like a Tracklok(sp?) doing it's "grunching" routine when the friction modifier is not sufficient to do the job on tight discs.
Strange that it would not do it before, but is starting to do it now. Usually it's doing it as soon as you start driving it with either the wrong lube, or one with not enough of the modifier.
Personally, if you do feel like that might be the case, investing in a tube of friction modifier might be a good thing. Even though the lube is full it sounds like, adding it while parked on a hill might let you get it in there without draining some of the old out.
Does your housing have a drain plug of any kind? Or is the housing stock, with just the Currie innards?

Paul

 

[Chiclassics]

How do you know its coming from differential and not something like ujoints or tires touching something ?

 

[DirtDonk]

Great point.
Especially that pesky double-cardan yoke setup can create some really unusual noises/vibrations/transients/wahoos throughout the vehicle.
I've seen/had/experienced several of them going bad over the decades, and not one of them gave the same clues that they were on their way out.
But your description of it only doing it while backing up was what really triggered my memories of not enough modifier in an aftermarket (Ford Motorsport) differential.

Paul

 

[gnpenning]

I did my Currie axles and rear end rebuild back in the early 90s. Things were waaaayyyy different then. Very few specific 4x4 places in my area and we just didn't have the disposal income that many seem to have now. Finding vendors and parts with no Internet ment were used magazines for our info sources. It also ment we generally did the work ourselves. So more than likely everything was owner installed with a huge difference in abilities. It wouldn't be a bad idea to pull your axles and see what you have inside. Before that try dropping the rear drive line and/or adding a modifier to verify where the noise is coming from.

Good time to do a fluid change so you may want to hold off on the modifier and verify if you have a clutch type diff. I know tru tracs were available then since I put one in my front end. Just don't recall if they were available for the 9". Probably were I just don't know for sure. Different time.

 

[dmoses42]

That sound in reverse sounds exactly like a tire catching a fender or fender flare. The '74 I had made the exact same noise. Mine was from worn radius arm bushings and having 33" tires and no lift (with fender flares, of course). May not be the case with yours, but that was my issue that sounded like that.

 

[Broncobowsher]

Do you know if it has a clutch type limited slip? The good thing is the additive for the clutches isn't harmful for non-clutch differentials. Many gear oils have the additive built in as default. No harm in putting the additive in.

 

[Scott McKelvey]

I'll stress my first points. This is not a new thing. I've been watching it for a long time now. Its very specific when it happens (i.e. not a u-joint or the double cardan joint), in particular the direction of the turn and the fact that it's only when everything is hot, and when there's no real acceleration applied. In my normal routine, this just isn't a scenario i run into everyday. Which is why I'm just posting now

I have no idea what's inside the 9", other than it's it's not an open diff or a ARB or e-locker. that's why i started this thread. I assumed it was a Torsen style diff, but I'm assuming torsen diffs would never make that noise, correct? i had no idea what was actually inside a diff like this, but it sounds like its a limited slip with clutch plates, like the Yukon trak loc.

And I should assume this might be a friction modifier issue. Someone asked, but no, i do not have a drain plug.

At first I thought it might be the brakes on the Explorer disk brakes (that's not the case),. It's definitely in the diff. What I was mainly trying to confirm is that a Torsen style diff won't make that noise, and that there are in fact friction style LSDs for the 9" (because the pictures all look the same pretty much: that small pointy snout. Sounds like the answer is yest to both of those.

an alternative, which I read one instance of was that there were bad bearings or preload, and the noise was shifting the carrier. But it's sounding like that's not my issue based on the # of years this diff has been in service and the symptoms.

 

[Torkman66]

I agree with what others have offered. Might not even be the rear end...could be drive shaft, wheel bearing, etc...who knows. But your instincts are right regarding thinking the rear end might be the source. As mentioned, first thing you should do that is cheap and a good diagnostic is put in friction modifier. I hade very similar noise in mine and although I had already added a bottle of modifier, I was told to add another and it completely solved my problem. Back up is now noise free and quiet. So that is where I would start. Grab a bottle and put it in and see what happens. Then depending on the result, we can go from there.

 

[Broncobowsher]

Jack up both rear tires. Leave it in park (or in gear with a manual). Try to spin one tire. If it spins with one hand effort, helical gear type. If it takes a couple hand and is stiff, friction based limited slip. That is a generic test, there are exceptions both ways. Torsen has a variant that has a preload in it. There are some clutch types that spin free at low speeds (although I can't think of one for a 9" off he top of my head).

 

[DirtDonk]

From your initial description, I did not realize that this had been doing it from the beginning.
Since it has been doing in a long time, then yes, by all means do what was suggested in the first and subsequent replies and add some friction modifier.
It can’t hurt, and can only help.
Even if it’s a non-clutch type limited slip, as was said already, it can’t hurt.
It’s much more likely to be a clutch type, limited slip than anything else, because back then the other types were not that commonly used, and most people didn’t spend that kind of money on their Broncos.
Even when they were updating to Currie stuff, they typically went with the basic (less expensive) limited-slip differential.

Since you don’t know what’s in there, as can’t know with hundred percent certainty until you pull it apart, how can it hurt to add the modifier?

And so another way of answering your other question, is that, yes, your assumption was correct. Helical gear styles, and lockers do not normally make these kinds of noises.
Lockers make noises, but they’re usually on the clunky side.
Just add the modifier, drive for another couple of hundred miles to let it soak in and do its thing, if it can, and see what happens.

 

[DirtDonk]

Some other thoughts.
Can you post a picture of your housing? Just to see if it’s an actual Currie housing or just standard Bronco housing with updated insides?
Maybe they offered both options back then?

Easy to fake Currie axles by putting decals on the end, but not so easy and (not likely) if they’re actually accompanied by Explorer disc brakes.
But Explorer disc brakes were a rarity 30 years ago, so more likely narrowing it down to the 20 years or newer range.
I’d be interested to see the brakes as well.
Not unheard of back that, of course, but kind of rare. So maybe it was more in the 20 year ago range after all.
Which would probably be a good thing.

More of a curiosity than anything else, regarding the housing. Just wondering if it’s one of their reinforced banjo housings, or standard Early Bronco.

 

[73azbronco]

sure you have modifier in it, enough? Like a dead month old salmon on your driveway sure?

 

[dmoses42]

Not sure if a Spartan Locker would cause this, but it seems like it could cause the chattering, and if it only does it when its hot, it might not like whatever weight gear lube is in it.

https://shop.broncograveyard.com/Ford-9-Inch-Spartan-Locker/productinfo/22382/

 

[Chiclassics]

Again trying not be be a Smart *** but how do you know its coming from rear end? The reason I say this is sound is an interesting thing when you sit inside car and hear it , it reflects from weird angles and make you believe source as something that is not. Did you have someone lie on ground next to it and confirm that is the case? or did you lift the vehicle up and recreate this scenario to pinpoint the noise is from the rear end? Because from an engineering standpoint it is possible this to come from anywhere from your hubs to transmission to transfer case to drive shaft - the entire drivetrain. And there is something about your turn to passenger side making it more prone makes me believe something on your passenger side is binding (disc brakes? , hubs?). So i would still make sure that is right diagnostic in first place. One thing i would do is lift up vehicle in neutral and turn to passenger side and rotate front tires to check off any such binding. Then have someone else confirm the sound who is outside of vehicle to rule off any sound reflections.