Roaring Rear End - PeachParts Mercedes-Benz Forum

Tpawlik · #1 10-07-2010, 09:28 AM

I recently replaced the all four subframe mounts, all three diff. bushings, shocks, rear diff. fluid and sway bar links. Car handles great now. However, It sounds like I am being chased by a freight train. It sounds like harmonic noise. It is so loud it drowns out the radio. It is not as loud when I let off the accelerator and it starts to go away above 80 MPH. The only thing I see from the shop manual pics. that I did not do right was the shim on the front diff mount. It is supposed to be between the diff and the bottom of the two piece bushing. I have it on top of the subframe under the top bushing.
I am going to move it tonight.
Could this one piece being out of place be transmitting all this noise through the subframe and body?

vstech · #2 10-07-2010, 10:11 AM

if there is rubber between the shim and the mount, then no. but if the shim contacts steel and could connect the vibrations to the frame, then yes.
look over every connection, and see if ANY steel is connecting together. every thing in the subframe needs to be isolated with rubber.
also don't forget that overinflated tires or old rubber can amplify road noise and SERIOUSLY create a roar.

cirrusman · #3 10-07-2010, 10:12 AM

If it's the one thing you think you did wrong, I'd say yes... sounds to me like in the process the driving shaft might have lost a balance weight...

vstech · #4 10-07-2010, 10:14 AM

also, you are SURE you filled the diff right?

Tpawlik · #5 10-07-2010, 10:38 AM

I lost most of the fluid though the sensor that bolts on the side off the diff. when I was wrestling with one of the axle nuts. I took the subfram completely out of the car. I removed the higher of the two drain plugs and added one quart of 85-90 weight gear lube. It was just starting to spill out when the bottle was empty. I forgot to list that I replaced both half shafts because the boots on the originals looked like they may split at anytime. They do not clunk and I do not think they are the culprit.

vstech · #6 10-07-2010, 10:43 AM

what did you replace the axles with? some rebuilds are crap, and could easily cause the noise.
do you still have your originals?

Tpawlik · #7 10-07-2010, 11:44 AM

The only ones I saw for sale were the FEQ brand. After I installed them I realized they were made in China. I was going to toss the old ones but thought better of it. I still have them. I really don't want to remove and replace that half shafts AGAIN. It was not that hard when the subframe was sitting on the workbench. But, if moving that shim doesn't stop the noise, I guess the half shafts will be next.

vstech · #8 10-07-2010, 12:17 PM

if you put in Chinese axles, you WILL be replacing them again.
I give it 3 months.
I'll let you know when I have the boot gun available to go back into the rental pool. as long as your boots have not torn and leaked out the oil, they are FARRRRR superior to the chinese junk!

Tpawlik · #9 10-07-2010, 12:45 PM

I should have known by the price, it was too good to be true! The originals were not leaking and were not making any noise. I only replaced them because the boots are badly cracked and I had the whole thing apart anyway. I have heard some people have not been able to stretch the boot enough to get them on. If I had the boots and the grease, I quess I could take them to a transmission shop to reboot them?
How hard a job is this and on average how long should the innards of the originals last?

Billybob · #10 10-07-2010, 01:54 PM

Quote:

Originally Posted by Tpawlik

I recently replaced the all four subframe mounts, all three diff. bushings, shocks, rear diff. fluid and sway bar links. Car handles great now. However, It sounds like I am being chased by a freight train. It sounds like harmonic noise. It is so loud it drowns out the radio. It is not as loud when I let off the accelerator and it starts to go away above 80 MPH. The only thing I see from the shop manual pics. that I did not do right was the shim on the front diff mount. It is supposed to be between the diff and the bottom of the two piece bushing. I have it on top of the subframe under the top bushing.
I am going to move it tonight.
Could this one piece being out of place be transmitting all this noise through the subframe and body?

After doing the things suggested and before replacing parts, try to ascertain the condition of the rear wheel bearings. You can do a visual with the usual prying and prodding and you can maybe learn something driving it. Does the roaring seem speed dependent? Find a shopping mall, warehouse or similar building where you can drive along for a distance with the window open listening. You can often hear "bad" sounds as the bounce off the wall you're driving parallel to. In a large empty parking lot you can also try "loading the rear bearings by driving in large circular arcs, which will load the outer wheel's bearing and possibly cause it to announce its impending failure.

A failing bearing will often time "roar" at speed and will vary in intensity at certain speeds. They are kind of hard to diagnose without obvious failure and without taking things apart some.

Tpawlik · #11 10-07-2010, 02:19 PM

I did try to pry up on both tires with a long bar and tried to move both side to side, like you would check the front wheel for ball joint or tie rod play and did not get any movement. Plus this noise is new, only since I had the subframe out of the car.

Billybob · #12 10-07-2010, 06:02 PM

Quote:

Originally Posted by Tpawlik

I did try to pry up on both tires with a long bar and tried to move both side to side, like you would check the front wheel for ball joint or tie rod play and did not get any movement. Plus this noise is new, only since I had the subframe out of the car.

Is this the same vehicle where you suffered the axle nut problem? Did the axle slide out easily once you removed the nut?

The 124/129/201 rear bearings are a sealed unit pressed into the wheel carrier and onto the wheel hub, it is difficult to observe radial or lateral movement unless things are very worn. Movement early on will be axial, more inward and outward than up or down or side to side. With the rear end off the ground and the caliper off the rotor you can sometimes detect the in & out movement by bolting a tireless rim to the hub and grasping that to push and pull at the hub (you can use a rim with a tire but it's harder to grasp and the tire flexes making it harder to detect movement from the hub). Any movement in or out is the bearing.

But unless I observed definate bearing movement I would install one of your old rear axles at a time and see if that eliminates the noise, if the noise disappears it's the new axles if not I would really start to suspect a bearing.

These sealed units last a long time but when the start to fail it's usually a steadily accelerated failure.

babymog · #13 10-07-2010, 02:29 PM

Does sound like wheel bearings, and they don't necessarily have play if they're bad.

whunter · #14 10-07-2010, 07:40 PM

Quote:

Originally Posted by Tpawlik

I recently replaced the all four sub-frame mounts, all three diff. bushings, shocks, rear diff. fluid and sway bar links. Car handles great now.
However, It sounds like I am being chased by a freight train.
It sounds like harmonic noise.
It is so loud it drowns out the radio.
It is not as loud when I let off the accelerator and it starts to go away above 80 MPH.
The only thing I see from the shop manual pics. that I did not do right was the shim on the front diff mount. It is supposed to be between the diff and the bottom of the two piece bushing. I have it on top of the sub-frame under the top bushing.
I am going to move it tonight.
Could this one piece being out of place be transmitting all this noise through the sub-frame and body?

Yes, it is possible for an incorrectly assembled mount to act as a noise transmitter/amplifier.

.

Tpawlik · #15 10-07-2010, 09:09 PM

Thanks for everyone's input. I moved the shim to the location shown in the shop manual and the noise disappeared.
I still have some rear tire noise. The PO had recently put new Continental Conti-Touring tires on the front and had older Yokahama's on the rear. I noticed when I put the tires/wheels back on after all the subframe work that the tires are directional. They were on backwards. I turned them around so they are rolling in the right direction. Would that matter?

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