I apologize in advance if there is an answer to my question which has previously been posted on this site. I searched the archives for quite a while and could not find the same symptoms. I have a 1984 300 SD with an automatic transmission and 170,000 miles on it. It is only at low speeds like 25 and below, but especially at 10-15 mph that I hear/feel this faint noise. It also only occurs after the car has warmed up. From the driver's seat, the noise/vibration comes from the vicinity of the transmission. The noise/vibration is a dull sort of knocking, with no metallic noise or any kind of thumping. There are no vibrations or noises at highway speeds or even speeds of 30 or higher. I have changed all the rubber exhaust hangers to start with the easy possibilities. No change. I have visually inspected the exhaust mount at the front of the transmission (good condition), the motor mounts (no cracks, not compressed), the transmission mount (seemed slightly spongy, but no cracks, and it seems to be designed to move around somewhat), the flex discs (completely intact, but some slight stress cracks along a rib joint), and the diferential mount (seemed to be in excellent condition). I got a look at the center bearing and support from the viewpoint of the rear axle with a flashlight, and the rubber support seemed in good condition, but I could not really determine if the center driveshaft bearing was damaged or not. I guess I would have to take off the heat shield to see it properly.

So anyway, all these various components seem to be in no need of replacement, except the transmission mount and the front flex disc. But as I said, the transmission mount seems in excellent condition, it just flexes a little if I push on the transmission, but this seems normal. And the cracks on the front flex disc are only visible if you push on the outer edge of the flex disc and distort the disc, revealing a very shallow crack on the joint that surrounds one of the bolts.

This noise/vibration is really quite faint, and I am wondering if I even need to worry about it. The car drives absolutely flawlessly, so there are no symptoms that actually affects daily driving. I just hear/feel it everyday and I want to make sure its not something that is a part that needs to be replaced which I can take care of.

Sorry for the lengthy post, and thanks for any help,

Gilbert

mount sounds like the suspect. not strong evidence but you seem to think it may be on its way out. if it is letting the back of the tranny down it may be the exhause is hitting something.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Thanks Tom, for the reply. I took the car to my independant mechanic today to diagnose the noise, and of course the noise would not show itself during the test drive, so I guess I will just wait until the noise gets really bad.

Gilbert