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Discussion Starter #1
When in the morning I start driving backward I can hear very scary sounds like someone hits the bottom with a 5-pound hammer.

What else I've noted:
1. If I move straight back I won't hear the hits with a very high probability
2. But if I turn the wheels - -even a bit - I'll always hear the sound.
3. I've never hear the noise while driving forward.
4. If I drove far enough I can bravely drive back - there will be no such the noise.

Did anyone know what is the origin of the sounds?
 

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What year, engine, 4wd drive system and how many miles do you have?

What you're describing is a "Clunk", I'd suspect from the drivetrain.

The front differential is mounted by 3 rubber mounts, that are known to wear out and allow clunks. Of course if your Commander is RWD only, you won't have this equipment.

When was the last time you changed axle and XFR case fluids?
 

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Discussion Starter #3 (Edited)
2008, CRD, QDII. 81K.

Let me clarify the "clunk" word. It is "clunking" when rubber parts that should prevent metal parts from contacting each over are getting worn and one metal part starts beating the other one (on the road humps etc). Am I right? If yes, the sound that I can feel (not only hear) definitely is not the clunk.
It rather reminds me a great supporting bearing (like in McPherson suspension) that is full of sand, attached to the huge spring and what I am, nevertheless, trying to turn.

I have changed the axle fluid less than a year ago (< 10K), the transfer case's fluid was changed a week ago, the gearbox' fluid was changed a couple of months ago.

P.S. The front pads are pretty worn.
 

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2008, CRD, QDII. 81K.

Let me clarify the "clunk" word. It is "clunking" when rubber parts that should prevent metal parts from contacting each over are getting worn and one metal part starts beating the other one (on the road humps etc). Am I right? If yes, the sound that I can feel (not only hear) definitely is not the clunk.
It rather reminds me a great supporting bearing (like in McPherson suspension) that is full of sand, attached to the huge spring and what I am, nevertheless, trying to turn.

I have changed the axle fluid less than a year ago (< 10K), the transfer case's fluid was changed a week ago, the gearbox' fluid was changed a couple of months ago.

P.S. The front pads are pretty worn.
The QDII has LSD's and if the axle fluid wears out or they didn't use the correct or correct amount of friction modifier to allow the clutch plates to slip well enough, you get noise and binding in low speed turns. Check your O.M., I think the severe schedule (maybe the normal for QDII) is every 15k miles. If the correct additive wasn't used that might be the problem or maybe the fluid was contaminated?

When you turn at low speeds, there is a lot of torque in the driveshafts and wheels moving at very different speeds, differentials turning, etc, compared to just backing up straight; so if there is any lash/free play in the driveshafts or the mounts for differentials,

A clunk, is a clunk, it's a noise of something impacting something else, doesn't have to be metal on metal, it just has to be hard enough to make a loud impact noise. Worn out mounts are often metal on rubber, there is just enough movement with momentum behind it, it sounds like a hammer banging on the floor.

What you're describing sounds like the steering bearing plate in the macpherson strut, and that would make a grind noise, NOT a clunk, unless you're talking bearings so worn out they seize and clunk. Do you have a grinding noise?

I thought of one else, the Sway Bar bushings, it could clunk turning as you back out of the driveway, but it would also clunk when driving normally, something you haven't said is a symptom.

The sway bar twists as the body leans, thus making a spring force to resist the twist. That will push up and down on the bushings that mount it, and up/down pressure reverses as you turn the opposite direction. The sway bar bushings can wear, the hole the bar goes through elongates and lets the bar move up and down some. So when you reverse forces on the bar, it quickly is bounced from one end of the elongated hole to the other and makes a clunk when it hits the opposite side.
 

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Discussion Starter #5 (Edited)
Thanks for such the detailed response. I am happy we both understand the same words in the same way. No, the mounts aren't clunking. Everything is quiet, all the bushings are almost new (I replaced them ~8K ago or so).

The fluids are ok, I know it for sure. I have replaced all the bearings/seals etc last autumn.

And I checked all the bolts of the suspension just a week ago. Tightened them up to the spec's requirements.

Frankly, I just can't imagine what else can it be. Bad gear backlash value would destroy the differential during this week (I took a long ride that was almost 2.5K miles), but it works well and silently all the time. Worn mounts would noise every time I give them extra moment of flexion, but they are silent even on the humps. Worn cv-joints? Don't know... I never replaced them but the boots are ok and they produce no sounds during sharp turns. Lower arms are all new, rod ends too, upper arms are functional and tight.

I remember that the first time I've heard the sound was when I replaced front shocks. It was a couple of years ago. Though, I don't know what relation it has to the issue. I can't find the source of the noise and it literally kills me. I spend more and more time under the car instead of driving it.
 

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Is your exhaust flexing and hitting the floor board? Worn or broken engine/trans mounts? In reverse the torque will flex in a different direction than in forward.
 

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Discussion Starter #7 (Edited)
Is your exhaust flexing and hitting the floor board? Worn or broken engine/trans mounts? In reverse the torque will flex in a different direction than in forward.
No. Everything (except for the fuel tank which is corroded and fixed by the Arbo Steel) is in very good condition.

The only thing I didn't check yet is the steering gear mounts. Maybe bolts loosen?... It would explain everything. Will check it and write here.
 

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Have you checked the top transfer case bushing? That causes clunking noises commonly on these vehicles, and cannot be checked with the xfer case installed AFAIK.
 

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Discussion Starter #9
Have you checked the top transfer case bushing? That causes clunking noises commonly on these vehicles, and cannot be checked with the xfer case installed AFAIK.
Frankly this is the very first time when I hear that the transfer case has any bushings. Isn't it hardly bolted to the gearbox what is being supported by rubber pillows?

Could you please show me the bushings at the picture attached?
 

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