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?

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 157k miles
06 E320 CDI "Rutherford", Black on Tan, 175k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 70k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

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.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

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.

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Ed
-1984 Mercedes 190D 2.2 5-speed gray market(bought@30,000 miles) (Sold back to original owner@170,000 miles)
-1999 Mercedes E300DT (245,000 miles)
-1999 Mercedes S500 Grand Edition (80,000 miles)

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

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 157k miles
06 E320 CDI "Rutherford", Black on Tan, 175k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 70k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

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.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 157k miles
06 E320 CDI "Rutherford", Black on Tan, 175k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 70k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

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.

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1998 E300 turbodiesel

America's Rights and Freedoms Are Not The Enemy!

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

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 157k miles
06 E320 CDI "Rutherford", Black on Tan, 175k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 70k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

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.

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Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)

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.

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k

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.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 157k miles
06 E320 CDI "Rutherford", Black on Tan, 175k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 70k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

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.

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1998 E300 turbodiesel

America's Rights and Freedoms Are Not The Enemy!

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

__________________
14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 157k miles
06 E320 CDI "Rutherford", Black on Tan, 175k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 70k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife