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I hope its ok to ask this here since it is a different vehicle. I've asked my Mustang questions here and got good answers so hoping to get a better idea on this problem. I have a 99 Mountaineer with the 5.0, 2WD, AODE transmission. The drivetrain is similar to what some are running in their mustangs if they put the AODE in.

I've been getting a clunk noise when accellerating from a complete stop. I am certain it is in the drive train.

I have already replaced the u-joints and that didn't fix it. When I sit in the back of the vehicle, I can hear the clunk coming from the rear. It sounds like it is in the differential.

When I had the driveshaft out, the pinion flange did not appear to move in and out. The driveshaft can be twisted a little with the wheels off the ground, but not excessively. With the wheels on the ground, it can't be twisted at all.

I can come to a complete stop and let off the brake gently to avoid the truck rocking back, and it will clunk about two seconds after rolling.

If I accellerate hard, it will clunk immediately. If I just let off the brake and let the truck roll, it will roll a few feet and then you can hear a light clunk.

Someone said it could be the transmission kicking down to 1st gear late, causing a clunk. But I can come to a stop, wait a minute, and it will still clunk when I take off. This allows plenty of time for the tranny to kick down. The tranny seems to be shifting fine though, up and down. It really feels and sounds like it's in the rear, but I have no idea where else to look.

Any thoughts what might be causing this?
 

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Possible control arms / bushings would be my guess
 

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That would be my idea. Also do you have torque boxes? Could be that too.


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I wonder if it isn't coming from a mount. What happens when you brake torque it? Do so in forward and reverse. Often times a transmission mount will clunk when brake torquing in reverse. Also, shifting from drive to reverse, back and forth, will generate a clunk from a worn mount.
 

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Same sound before and after the u-joints were replaced? When it was up in the air and you turned the shaft, did the noise sound like it was at the flange, or inside the diff itself? If its up in the air and you hear a clunk, i doubt its the suspension. If you feel up to it jack it up and have someone under the truck (block the wheels) while someone is in the driver seat working the gas and shifter. There will be less load on the drivetrain but it could still help in finding where the clunk is from. Mind the driveshaft and tires, obviously.
 

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Let me see if I can find my past threads. I have had the EXACT same problem, and I found it a few weeks ago. Give me a few minutes...
 

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Here's the thread that I started. Since I had replaced my rear end and all of the components and was still having the problem, I had thought it was my torque boxes.

Lo and behold, it was my rear upper control arm bushings. In this ten second video, my friend puts a crowbar between the bushing and the frame and gently pries. You can see the control arm slide back and forth, indicating the bushing is shot. I can't tell you how relieved I was to find this, because I had tried to reproduce the sound a number of times with the car in the air at home and couldn't, because I didn't have enough strength to make it seem as if it were under load. See video here:

https://picasaweb.google.com/104039...hkey=Gv1sRgCLyOjbe14pXiMw#5820338833709618914

Try doing the same thing on each of your control arms and seeing what happens. Given your explanation, it sounds very much like what I was experiencing.
 

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Possible control arms / bushings would be my guess
That would be my idea. Also do you have torque boxes? Could be that too.


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I think he's taking about a mountaineer not a mustang. The mountaineer is leafsprung full frame of course.

It might be your diff. how many miles on it? Stock suspension?
To eliminate the downshifting trans hypothesis shift it into first gear and try to replicate the noise. If it still makes the noise it can't be the tranny downshifting because it's already in first.
 
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