Droning sound through chassis at cruising speeds under throttle - what to check? - PeachParts Mercedes-Benz Forum

bodyart27 · #1 11-21-2005, 01:26 PM

When at a cruising speed, I've noticed this past week or so that there is a BIG sound difference if I'm on the throttle cruising or if I lift the gas. When I'm holding the throttle down there is a heavy droning sound (there was always a little, but I don't remember it being that bad). When I lift all you really hear is the wind, engine idle, and tire noise. When I get back on the gas the heavy drone kicks in.

I did motor mounts about 20-25K miles ago (and transmission mounts), but I have not replaced the little engine shocks. Do you think they could be engine mounts already or should I check something else?

If I watch the engine with the hood up (foot on brake obviously), as I go from Drive to Reverse the top of the engine does pitch some, but I would say the shake at idle is not that bad. Under highway load, something sure is resonating. I just had the car aligned when I did tie rods so I'm not feeling anything through the steering - she's driving straight as an arrow. I do feel some vibration through the shifter.

Thoughts?

rg2098 · #2 11-21-2005, 02:30 PM

Sounds like the same thing my car does although describing sounds is difficult. I have all new mounts, all new front suspension / alignment, new driveshaft support bearing, pretty much every wear out part, etc. It may just be the nature of the beast.

t walgamuth · #3 11-21-2005, 09:37 PM

thought was motor mounts. i would have a good look at them. the engine shocks are cheap so i prob would change them too but i don't think that they are a likely culprit. then too it could be a twisted exhaust pipe that is getting metal to metal contact. i would also check the tranny mounts to be suee a bolt hasn't come loose and allowed it to drift off to one side.

tom w

bodyart27 · #4 11-22-2005, 09:18 PM

I looked up when I did the motor mounts (I just keep a running log), and the car was at 181K miles. The car now has 207,400.

Sound about the right time to replace the Mounts? How much life do you see out of these things?

t walgamuth · #5 11-22-2005, 10:44 PM

but if you leak fuel on the drivers side it will rot away much quicker.

tom w

#6 11-23-2005, 02:00 AM

The motor mounts are most likely just fine.

What sort of condition is your exhast system in?

Ken300D · #7 11-23-2005, 10:21 AM

I'll mention another possibility based on my own experience with the 1982. When you replaced the engine mounts and especially the transmission mount, you raised the transmission a bit.

The driveshaft has only one universal joint and it sits in one position most of the time and wears in that place. When you lifted the transmission slightly, that wear point is moved slightly and now the droning is being caused by a new wear point within the universal joint.

As far as the driveshaft goes, the support bearing tends to make a low-speed rumbling when worn, or possibly a squeal when really worn. A droning at highway speed under power application tends to point to a universal joint issue.

Not much you can do about the universal joint issue except have the whole driveshaft refurbished and rebalanced. I tried doing a u-joint replacement and was not satisfied with my work.

It's not designed to be easily replaceable.

If you go as far as to remove the drive shaft, you will probably be able to feel the wear in the movement of the U-joint easily. There will be a "center detent" kind of wear spot right in the center where the U-joint gets "stuck". A non-worn U-joint shouldn't get stuck like that in the center but should move freely from side-to-side.

I kinda regret replacing the transmission mount in that car and on the other cars won't do it unless there is metal-to-metal contact. Then I can also plan on having to get the driveshaft rebuilt.

Ken300D

dlssmith · #8 11-23-2005, 10:38 AM

After checking some of the easier stuff already mentioned, I would look at the bearings in the rear axel and differential. They can be whiners. A cheap test for the diff would be to change the fluid and put in new synthetic. If there is noise back there, it might quiet it a bit.

lefttownlance · #9 11-23-2005, 11:05 AM

how about rear end problems in the past?
Have you looked at the grease or inspected play in the pinion gear? To much end play will cause a moaning sound.

pawoSD · #10 11-23-2005, 11:15 AM

Any car has a difference in sound on or off throttle, my car has a dull "growl".....our van more or less "groans" and my uncles cavalier sounds the same pretty much. Also it depends on the speed.....sometimes I'll hit an RPM/speed where the frame resonates the engine, and its loud for a moment. All my mounts etc are new too. I like the growl sound though. Sounds Tough.

ykobayashi · #11 11-23-2005, 06:47 PM

Hi,

Yeah, I just noticed my car has a low roar under power - like a worn tranny or diff that has gotten louder this week.

I'm going to jack up my car and spin the wheels, inspect the u-joints and shake the tranny (1982 240D 4speed) on the center mount and see if it is anything easy. The car hummed a few weeks ago but I notice it is louder now.

I have very slow leaks in the seals on my diff and tranny. The next step is to check my fluid levels. They leak so slowly though I cannot see how they could have drained out in a year.

I'll post if I find anything easy/hard.

Cheers,
Yoko

t walgamuth · #12 11-24-2005, 10:40 PM

is low it can definately make noises. if it is leaking better keep an eye on it. low level can trash bearings (ask how i know). seals are a lot cheaper to do than tranny bearings!

tom w

Elktonjohn · #13 11-25-2005, 12:02 AM

What timing, I recently replaced the carrier bearing assy and noticed the stiff detant area in the U-joint and wondered how long that would last...and how to R&R the bugger. Now I'll just leave it and forget my next job also...R&R the trans mount...looks like hell but it's hangin in.
Also get a nice sounding howl from the rear grears and want to change the gear oil (267K is probably about time) but I'm just praying it's as easy as doing it on a Chevy...are there any tricks or tips in getting level right or is it just up to the level of the fill-hole? I've not even looked at it ( rear housing ) yet, so feel guilty about asking but everything I fix on this old girl teaches me how great the differences are between GM and MB and now that I don't have to worry about the mount my weekend is about shot without something to fix.
Thanks, take care and enjoy!!

ykobayashi · #14 11-25-2005, 07:42 PM

Hi,

I got the car up and spun the shaft around and spun the wheels front and back to see if the bearings ground. They all sounded okay. However judging from the posts here, I think I'm going to keep an ear on the thing in case the u-joint has a stiff part like people say here. Also spinning it around doesn't tell me how they perform under load.

Dang t walgamuth, when the car was new to me the tranny fluid was low and it has a grinding noise just when I start it (idling) that goes away when I push the clutch down. If the car is warm it doesn't make this noise. Could this be the bearings you're talking about? Is it the input shaft bearing?

Well, while I was under the car, I should have pulled the tranny plug and checked the fluid level but I got lazy and decided to come into the warm house and see what was up on the diesel discussion instead

. I'm such a dope, I gotta get back out there and check the fluid level rather than guess its level!

The car only seems to roar at 65mph +.

Okay, thanks for the tips,
Yoko

Quote:

Originally Posted by t walgamuth

is low it can definately make noises. if it is leaking better keep an eye on it. low level can trash bearings (ask how i know). seals are a lot cheaper to do than tranny bearings!

tom w

ykobayashi · #15 11-25-2005, 09:40 PM

Hi,

Okay, got off my butt and checked the fluid in the diff and tranny. Plenty in both. The leaks are indeed quite slow.

So the noise I experience is like that originally described in the thread, high speed only 65mph+ and only under load (goes away when I hit the clutch).

The u-joint appeared to have little or no play in it, but when they do, they are really blown. This stuff in the threads about the worn detente makes a lot of sense...,

So let me get this straight, I cannot just pull the shaft, press out the joint and press another in from the local parts supply? What is so "unrebuildable" about this shaft? Do I have to take it to a special shop or do I have to buy a rebuilt?

From the forum it looks like I should spring for the rebuilt.

Cheers,
Yoko