I have a 1981 300SD (126 body). I am experiencing a knocking sound eminating under the left rear seat. The frequency of the noise appears to be drive shaft related rather than axle related. The noise is also resonant such that it appears only at certain speeds and disappears at other speeds. The noise appears whether I am going forward or in reverse.

Could this problem be a bad rear flex disk?

Any other ideas?

Thanks.

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Tom Evans
69 280SE 91K (wish I still had it)
73 220D 98K
78 300D 134K
78 300D 185K
81 300SD 301K (still have)
82 300SD 321K (still have)
84 300D 268K
84 6.2 Diesel GMC Suburban 225K (a whole different story!)

hard to say, but if you even SUSPECT it is, you need to investigate. any cracks, or threads sticking out it's GOTTA go before your car gets seriously damaged.
more likely it's the center driveshaft bearing or the center u-joint.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

You are correct.

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You're four times

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more likely to

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a cell phone.

www.kiva.org It's not like there's anything wrong with feeling good, is there?

You should study the pictures of BAD flex disc's found through this thread.

Driveshaft, Axle & Wheel bearing


.

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1985 300SD
1983 300D
2003 Volvo V70

https://www.boldegoist.com/

yeah. read all those threads and notice one common theme...
no symptoms. "it just started making this horrible thumping and the car would no longer move forward..."
get under your car and look over both discs and verify condition.
then pull the driveshaft and look at the center bearing.

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Symptoms = you'll feel something...

If you do not know when they were last replaced, might as well, if you've determined they're suspect (potential small cracks, etc). My 300D, 190E and mother's 300TE all have new flex discs - just to have peace of mind.

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http://farm8.staticflickr.com/7271/7...144c3fc1dc.jpg

Best to do both flex disks, center support, bearing and bushings the 1st time. I found receipts where PO changed 1 flex disk. The rear disk has cracks.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

Thanks, guys. I'll be out of town until just after Christmas (not driving this car!) and will check out the drive shaft thoroghly at that time. I still can't figure why the noise appears to be coming from the left of the drive train. I guess the drive shaft is just a good ventriloquist!

I'll let the forum know what I find out.

__________________
Tom Evans
69 280SE 91K (wish I still had it)
73 220D 98K
78 300D 134K
78 300D 185K
81 300SD 301K (still have)
82 300SD 321K (still have)
84 300D 268K
84 6.2 Diesel GMC Suburban 225K (a whole different story!)

A rumbling resonance on 'float' (neither pulling or coasting, but maintaining speed on level ground) accompanied by a slight whine indicates worn (loose) pinion bearings...FYI

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1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership