Mercedes: 1989 300SE - rumbling/vibration only on acceleration between 30mph and 45mph, valve cover gasket, oil seepage

Question
Thank you very much for your prompt response!

I do have this follow up question:

The motor mounts were replaced about 60,000 miles ago along with all of the suspension parts.
All four tires were replaced about 20,000 miles ago. During the last maintenance, I did inquire
about wheel bearings but the mechanic didn't say if they were bad.

As typical for W126 auto, I do have minor oil seepage around the valve cover gasket, but could
these lead to plug fouling, which may be the cause of my rumbling/vibration/shake? Also could
vacuum modulator to the transmission be a possible cause? If the cause is a bad differential,
wouldn't I see sense rumbling/shakes at all speed?

Again, thank you very much for your work, and I eagerly await your response.


-------------------------
Followup To
Question -
I have a 1989 300SE with 154000 miles, overall in great shape!

However, the car experiences shakes/rumbling/vibration only
when I accelerate from 30mph to 45mph. From the stop to the
30mph point, the engine is smooth, and from 45mph and up,
the engine runs smoothly! Also, when I coast from higher speeds
(say 65mph) all the way to stop, the engine is running smoothly.
Idle also appears to be smooth, albeit a slight vacuum leak
possibly (soundwise anyway).

I've had a tune up done about 10,000 miles ago (plugs, wiring,
distributor rotor and cap), and front flex disc, 20,000 miles ago,
all within the past 1 1/2 year.

TIA.
Answer -
Best avdice:
Replace the engine mounts before you touch anything else, they are past due, and probably
collapsed = engine resting on body.

A speed related sound could be:
#1. A bad tire.
#2. A bad wheel bearing.
#3. A bad differential or axle shaft.
These need profesional inspection.

Answer
Best guess, without hearing the sound:

Rotate and balance the tires first.
Inspect the drive shaft support bearing, it can cause a sound similar to what you describe.

When where the cap, rotor, wires and plugs replaced?

Oil leak from the valve cover can wreck the plug wires = cause fouling.
Raise the car, and inspect every inch of the exhaust system, and body for witness marks = rubbing due to engine torque.

Inspect the rear sub frame mounts, and differential mount.

Read these threads:

126 rear shock change
http://www.mercedesshop.com/shopforum/showthread.php?p=1031835#post1031835

Rear sub frame Bushings W126 1985 300SD DIY
http://mercedesshop.com/shopforum/showthread.php?p=796976#post796976

Trailing Arm Bushing W126 1985 300SD DIY
http://www.mercedesshop.com/shopforum/showthread.php?t=111052

Anyone ever broke a rear subframe bushing bolt?
http://mercedesshop.com/shopforum/showthread.php?p=461406#post461406

Steering squirrelly W126
http://www.mercedesshop.com/shopforum/showthread.php?p=778649#post778649

W126 subframe mount clarification needed
http://mercedesshop.com/shopforum/showthread.php?p=370569#post370569

W126 Ride quality
http://mercedesshop.com/shopforum/showthread.php?p=543139#post543139

W126 1983 300SD rear shock & spring R&R DIY
http://www.mercedesshop.com/shopforum/showthread.php?p=1121537#post1121537