anyone want to surmise (~guess) what the knocking noise is? - PeachParts Mercedes-Benz Forum

Junkman · #1 10-08-2011, 10:13 PM

I'm about 30 mi from home & I hear a faint rotational knocking noise. It varies with vehicle (85SD) speed and goes away when going down hill with light throttle. It seemed to be getting progressively worse.

It is dark out and we're in the country with no flash light except for the app on my phone. I pull over shake the rear tires then pull on the rear axles. All seems nice and tight.

I start driving & the noise is gone but reappears when we're almost home. I'm thinking axle, drive shaft or perhaps something strange with the brakes.

Brakes were serviced within the last 15,000 mi - new rotors, pads, reman calipers & master cylinder.

I'll go look in the morning but will wonder all night. Any guesses about what it is??

Aquaticedge · #2 10-08-2011, 11:01 PM

Carrier bearing, something caught in the drive shaft, torn boot leaked it's fluid bad CV axle, tire loose or out of balance warped rotor (anything is possible) Flex disc's failing differential mount or other type of bushing failure.. Depends on where the knock is coming from

leathermang · #3 10-08-2011, 11:49 PM

Next time you hear it...
back up a few yards..
if it stops for a while...... very likely the CV joint on one axle...

kerry · #4 10-09-2011, 09:28 AM

I'm going with CV joint especially if it varies with throttle input downhill.

surveyguy · #5 10-10-2011, 12:51 AM

U-joint my guess. louder with more throttle = more force on the u-joint. doesn't change frequency, just makes it louder. u-joints typically act up at the same speeds each time (may even occur at a few different speeds. 15mph then again at 30mph)

Stretch · #6 10-10-2011, 04:22 AM

Mud in your tyres

Mud In Your Tires - YouTube

I know I've posted this before - but the old ones are the good ones!

vstech · #7 10-10-2011, 01:31 PM

heh, love that movie!
as a mechanic, of course, knew from the beginning the car couldn't have burned out as described...

Stretch · #8 10-10-2011, 01:36 PM

Quote:

Originally Posted by vstech

heh, love that movie!
as a mechanic, of course, knew from the beginning the car couldn't have burned out as described...

Did you really? My knowledge of US tin isn't that good - it was a total cinematic surprise for me!

Junkman · #9 10-12-2011, 11:20 AM

Unless MY carrier bearing is supposed to have play, it is the carrier bearing. $22.95 Rein brand. I think I'lll pull it & check the bearing store.

Besides u-joints & flex disk, is there any other obvious "while I'm there"?

There is an excessive (at least some) fluid drips near the back of the transmission. I'll look at that too. My preference is dry but that may take a lot of effort. I'm hoping to do other chores & not wrench much until spring. We'll see.

vstech · #10 10-12-2011, 11:35 AM

Quote:

Originally Posted by Army

Did you really? My knowledge of US tin isn't that good - it was a total cinematic surprise for me!

yeah, I watched the entire movie expecting the burnout to be a movie goof, or a plot twist, glad it was a plot twist.
I didn't catch the IRS part, but still knew no pos convertible would have posi!

surveyguy · #11 10-12-2011, 12:01 PM

carrier bearings should not have play. however, they are encased in rubber which will have some "give" to it.

Junkman · #12 10-12-2011, 02:10 PM

Quote:

Originally Posted by surveyguy

carrier bearings should not have play. however, they are encased in rubber which will have some "give" to it.

I have the drive shaft out. All of the rubber has deteriorated including the support for the shaft. I'll put in the order parts after checking the flex disks better.

Edit: Both flex disks show signs of cracking. The plastic centering bushing also shows signs of wear. I'll replace all.

The transmission leak is cause for concern but at least I'm warned.

surveyguy · #13 10-13-2011, 10:44 AM

glad to hear. wish that was the story on my 300SD; instead it's the u-joint.