W210 Vibration/Noise when idling in reverse
 

The 98 E300 drives fine. But this morning I noticed a vibration/noise appears when idling in reverse. Completely goes away both when the car is moving and when idling in forward gears or neutral.

What should I investigate?

If it only does it in reverse, I'd be inclined to think it could be a heat shield on the exhaust or something that's moving when the engine shifts the opposite way. How old are your motor mounts? If they're prehistoric, it could be that when you put it in reverse, the engine moves just enough to let the support arm rattle against metal.

Check your trans mount. I have the exact symptom with my 99'E300 when the trans mount is going bad. I've had to replace it twice so far. Once at 110k and then at 240k. Solves the problem.

All mounts are original. Trans mount sounds like a likely suspect.

The other thing I've noticed is that it's more noticeable when the engine is cold. Not too noticeable when engine is warm.

Definitely does not sound like a heat shield kind of noise.

Assuming it's the trans mount, is there any urgency to getting it repaired? It would be nice if I could wait till I have it in for something else.

A small light vibration, or something heavy sounding? The shifter bushings go bad and will cause the shift lever to vibrate and rattle when in certain positions.

If a heavy sound, I concur with the mounts needing replacement. There are two engine mounts and one transmission mount. It makes sense to replace all three together. They are not expensive parts, nor are they difficult to replace, so even if you're paying shop labor, it shouldn't be too spendy. If they've deteriorated to the point of causing clunks or vibrations, they are *totally* dead and ready today for replacement, there is no benefit to waiting.

You will be pleasantly surprised with the improvement in the ride and feel! When I replaced mine, the entire driving experience became smoother and more pleasant, it felt like a new car afterwards.

Shifter bushings have been replaced and it is a "heavy" sound....so I'm assuming it's time to deal with the mounts.

Jamb a large screwdriver into the lower mount for the dampner, and see if its pooched. If so, dont buy a chaep to replace it, they dont last.

Motor mounts can be measured using an appropriate OD sized socket to dertermine how far they have collapsed. Search my name for more. Brain has expunged most old W210 procedures.

It sounds like your driver side motor mount is deteroauted. I'm willing to bet on that more than the tranny mount. Not saying that isn't worn as well, but more than likely the engine mount. My car did the exact same thing till I did both the mounts. They had 170k on them since the last replacement since 280k. They were just next to about collapsed.

As to brands...is it worth the extra money to buy MB branded? Can get for about $125 each. Or I can get Corteco and Febi for around $65-70.

I installed Corteco ~5 years ago and it still feels like new. Personally I would not pay double for the genuine parts in this instance. Unlike some of the others, Corteco is a reputable name brand.

Replaced both engine mounts...problem solved. Tranny mount was fine.