Knocking sound very distinct - PeachParts Mercedes-Benz Forum

Ears · #1 03-02-2015, 05:30 PM

I have a 82 240d. I drive my car about 200 miles a day 5 days a week.

The noise im hearing sounds like it coming from the transmission or carrier bearing.

It sounds like something is hitting the bottom of the car. or when the cv axel is going out in a fwd car and you make a turn in that direction and it makes a terrible rattle\knocking sound

It only happens when i have been driving on the freeway for over an hour

If it was already happening and i get off the freeway it will continue but at slow speeds it almost sounds like that exhaust rattling

I can hear it better with the windows up but a friend was driving behind me on the freeway and said he could hear it pretty loud

Sometimes it is 5-10 knocks at a time sometimes it lasts for 30 seconds to a minute at a time

NJ300sdl · #2 03-02-2015, 05:54 PM

May be the driveshaft center bearing.

Charlie Foxtrot · #3 03-02-2015, 06:15 PM

Sometimes a loose bolt(s) on the flywheel to torque converter or flywheel to crank can make a similar sound (sometimes confused for a transient rod knock, but behind the engine). You may have to put it up in the air & run it to find the noise. Good luck!

Ears · #4 03-02-2015, 07:47 PM

Do you know if the center bearing being bad can let the driveshaft cantact the bottom of the car?

ROLLGUY · #5 03-02-2015, 08:15 PM

Could be center bearing as others have suggested, or a bad flex disc. It is time for you to get under the car and do a thorough inspection. You don't want a flex disc to come apart while driving down the road.

Ears · #6 03-02-2015, 08:29 PM

If my flywheel was coming loose wouldnt it make the noise all the time not just somtimes when ive been driving more than an hour?

VT220D · #7 03-02-2015, 09:37 PM

Like ROLLGUY said look at the driveshaft bearing and flex discs first. A flex disc failure can ruin the transmission and transmission tunnel effectively totaling the car. As you can imagine this can create a dangerous situation on the road.

Other possibilities include a failing axle, a loose transmission output flange, the exhaust.

Either way if you can't say with certainty that your flex discs are perfect they should be checked.

Ears · #8 03-02-2015, 11:22 PM

Isnt the flex disc solid metal? What would i be looking for other than obvious cracks

Ears · #9 03-02-2015, 11:26 PM

Also can i check it without dropping the driveshaft? I have to drive my car to school 5 days a week

ROLLGUY · #10 03-02-2015, 11:42 PM

Quote:

Originally Posted by Ears

Isnt the flex disc solid metal? What would i be looking for other than obvious cracks

No, rubber. There are two flex discs- one at the trans, and one at the differential. Yes obvious cracks in the rubber around the sleeves (steel sleeves with the rubber molded around them that connect the discs to the flanges). Check the center support for cracks in the rubber as well. A bad U-joint can cause vibration and noise as well.

Carpenterman · #11 03-03-2015, 12:08 AM

Knocking in the rear that comes and goes has always been a bad axle in my experience.

Get the car up on stands and check everything mentioned, however. Especially check the rubber axle boots for tears and the axles themselves for play.

ykobayashi · #12 03-03-2015, 07:43 AM

Quote:

Originally Posted by Carpenterman

Knocking in the rear that comes and goes has always been a bad axle in my experience.

Get the car up on stands and check everything mentioned, however. Especially check the rubber axle boots for tears and the axles themselves for play.

Good tip. I couldn't think of any idea for the op but yes, I did have a bad cv in my 240d that did this. Cracked and neglected boot.

I could get it to stop knocking if I stopped the car, crawled underneath and moved the axel toward the wheel...it would move 1" in each direction. Totally dead cv. Good thing cheap half axels for the 123 are readily available.

rscurtis · #13 03-03-2015, 11:03 AM

Not the best way to go. Get a rebooted axle from vstech or send yours to CVJ in Denver for a proper repair.

Good thing cheap half axels for the 123 are readily available.

Diesel911 · #14 03-03-2015, 11:40 AM

Quote:

Originally Posted by Ears

Isnt the flex disc solid metal? What would i be looking for other than obvious cracks

The Flex Plate between the Auto Trans and the Engine is Metal and they also crack.

The Flex Disc is layers of some sort of fabric impregnated and covered with Rubber.

You can see the Rear one easlily but on mine it is hard to view the Front one without supporting the trans and moving some parts.

Diesel911 · #15 03-03-2015, 11:53 AM

Quote:

Originally Posted by Ears

Also can i check it without dropping the driveshaft? I have to drive my car to school 5 days a week

Reality. All vehicles fail eventually.

If you contiune to drive it like that the chance is extremely high that something is going to get damaged. The greater the damage the longer the vehicle will be unusable and the more expensive it is going to be to fix it; assuming that is possible.

You need some sort of back up plan to get to School; Friends, another Student, a borrowed vehicle, the Bus and so on.

I am facing this right not. My Wife was driving our Astro Van back and forth to work. I had not been driving the Van.

I got in the Van and it overheated and knocked due to a leaking Water Pump. My Wife claims she smelled gas but was actually smelling Coolant.

The point is She drove the Van without saying anything to me and the overheating caused a Spun Connecting Rod Bearing and ruined the Crankshaft.

The Engine and trans are out and the Engine is apart so I am sure on the Crank.
I have not checked the Heads for flatness yet.

My advice is the moment there seems to be serious trouble is to stop driving the Car till you know what is wrong and it is fixed and find another way to get to school.

Added: I forgot to mention not being able to use the Van is holding use up from moving into another House that I have. Also if I had not happen to drive the Van that day My Wife would have driven the Van till the Block was so damaged it could not have been reused. That in effect would have mean junking the 1992 van that when running good is worth about $2-$3K.