Rear end thunking when coasting - Page 3 - PeachParts Mercedes-Benz Forum

Danman · #31 11-04-2013, 09:30 PM

Driveshaft boot is new (along with bearing, support, ujoint, and centering sleeves). New flex discs, but it was doing it before with the old ones. Ill double check the Fd orientation. Checked trans tail shaft for noise/play by hand. Nothing noticeable. If I spin the tail shaft all I hear is something like fluid moving around. To add frustration, I can't replicate the noise running the car with the rear end lifted. Back calipers are tight, ill double check the fronts tho. The noise is throttle sensitive tho.

Is the purpose of ds boot just to keep debris out of the union?

cooljjay · #32 11-04-2013, 09:35 PM

If every thing is done....and raising the rear end and accelerating doesn't show the issue....and its not the brakes then it has to be suspension....check the shocks and mounts, rear differential mount, rear subframe mounts and control arm bushings....if everything checks out....it may be in the front and traveling to the back...

whunter · #33 11-04-2013, 10:24 PM

Quote:

Originally Posted by Danman

I hate to keep resurrecting this thread but surely someone has experienced something like this in some vehicle or another.

Here's a video (not mine), but the frequency and pitch of clunking is very similar. Mines not as loud (probably couldn't get good audio) and happens when feathering the throttle, or light throttle to maintain a certain speed.

To make things even more complicated, it sometimes clunks a few times rolling to a stop, much slower almost wheel revolution. Assuming its the same thing. Wheel bearing? Transmission gear wear?

http://m.youtube.com/watch?v=25B4SFZWDRs

Three thoughts:

#1. There is an exhaust support bracket to the rear of the transmission, if the isolation rubber bushings are damaged, it can make a noise that transmits to the resonator / muffler.

#2. Exhaust movement under load = weak or failed engine or transmission mounts are a common cause.
I have a similar sound on my 240D at the moment, the pipes shift position (grow) as they warm up. My system needs (when I have time) to be re-bent 13mm to eliminate the sound.
Not long ago, two tough ones came in, one turned out to be the fuel line brackets above rear sub-frame where missing, the other was muffler broken inside rattle/clunk at lower RPM.

#3. Loose transmission output flange.

This issue is becoming more common.
Bad 722.xxx-transmission output flange spinning on shaft

Transmission Problem & Fix

Output flange, 722.xxx transmission

************************************************

I will raise this topic only for safety

Flex Disc Orientation, safety and durability issue.
Flex Disc Orientation, safety and durability issue. - PeachParts Mercedes ShopForum

If your W123 is shaking down the highway. . .
If your W123 is shaking down the highway. . . - PeachParts Mercedes ShopForum

************************************************

.

Danman · #34 11-04-2013, 11:11 PM

I appreciate the replies. I will look into all suggestions offered. As I rule components out I will work my way to the front of the car.

My brother was riding shotgun with me the other day and I was manipulating the throttle to replicate the sound. He said he could feel it in the floor board. I rested my hand on the hump behind the armrest and could slightly feel the thumping. As mentioned before, there's no vibration or transmission to the steering wheel or brake/ accelerator pedals. For something to be moving that much, it shouldn't be so hard to find!

As I sort through this I will probably have questions about various components and conditions. I'll bring those questions back to this thread. Thanks again for everyone's input.