I know my guide rod bushings are shot and need replacement, I have them in the trunk... lol This obvious while driving slowly over bumps on the road however I have a single loud knock heard inside cabinet when decelerating and occasionally while shifting from 3rd to 4th gear. The noise is more pronounced around town when my speed just barely warrants 4th gear, transmission shifts I decelerate and I hear the single knock. Vacuum seems good, recently bypassed EGR with Kit from forum member and transmission is smooth with this exception. Could this be the guide rods i'm hearing? Could it be the sign of a failing transmission? Engine or motor mounts? Any obvious things for me to check right away?

Thanks,

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1987 300SDL 257K

suspension related. Check air cleaner rubber shock mounts.

So are you saying its not the guide rods making the noise?

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1987 300SDL 257K

have many components that can produce noises. Most of the clunks and knocks that are not engine-related are produced by worn rubber components in suspension, engine/transmission mounts and other shockmounts. In my case, the only item clunkiing under my hood was the air cleaner. I only mentioned it because it is so easy to check and repair.

It could still be the guide rods making the noise but it would be "noticed" around your feet, where they are located. Depending on how long the guide rod mounts have been severely worn, you may have other suspension parts starting to exhibit wear. This other single knock, what area of the car does it seem to eminate from?

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Sam

84 300SD 350K+ miles ( Blue Belle )

It happens when shifting from 3rd to 4th, sounds like its coming from the lower sode of the engine compartment, almost like it could be transmission flexing somehow not sure if that's possible. Checked the airbox rubbers and they are good.

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1987 300SDL 257K