Vintage Cars: Vibration in 300D Diesel Mercedes, vacuum modulator, diesel mercedes

Question
QUESTION: I have a 300D Turbo Diesel with 166,000 miles on it.  It is mostly use for road trips.  Recently upon deceleration from 75 mph, I experienced a lot of vibration and noise.  It has now started to occur at lower speeds.  It stops with mild acceleration. Any ideas?

ANSWER: Pat
need a little more info, sudden onset?
where is vibration - front of car or under seats, rear of car?
Any noise associated with it ?
all fluid levels checked - tranny/rear diff?
any leaks?
when were engine mounts changed?

deceleration noises can be rear end related or flex disc related ( drive train)

let me know the answers to the questions and I will see if I can help
Ron

---------- FOLLOW-UP ----------

QUESTION: Hi Ron,
It was a sudden onset (this car has been full of fun surprises).  It "feels" like it's coming almost directly under the passenger seat.  All fluid levels are fine - no leaks.  Engine mounts were changed about 3 years ago.  The sound is akin to back pressure on an engine when you downshift a manual tranny and release the clutch to slow the engine - deep throaty rumble.  There is also an odor coming from the beast after it's run (not the standard diesel odor).  It did blow the head gasket 3 years ago and filled the exhaust with oil.  The mechanic claimed that they drained the exhausrt system and all was well but the car has not had the same power as before.  I've experienced drive shaft u-joint issues on other (American) cars and this is not like that.  I have had vacuum issues with this beastie also - currently the rear passenger side door refuses to open or unlock, and there is a vacuum connection under the hood that sometimes comes loose and the beast will not shut off without hitting the stop lever and reconnecting the tiny little vacuum lines.  Thanks for any help you can offer.  I don't want this critter dumping me on the road again.

Answer
ideas
could be a manifold crack
vacuum modulator on tranny - does it shift smoothly or late and jerky?
flex disks on drive shaft ( no U joints on these ol gals- check for cracks)
I guess it is possible that tranny is heading south as in throw out bearing - how does the fluid look?/smell?

These car are vacuum sensitive, any leak will cause all sorts of issues
Let me know