FYI...
My motor mounts were quite worn. They were on the car for six years and 26,000 miles. Must've been lower quality mounts because that seems premature. The right was substantially more collapsed than the left.

Unfortunately, the engine noise remains. Based on the sound, I still suspect the turbo charger or something related to the intake/exhaust manifold.

Plan to take it to an independent technician week after this one to be diagnosed.

From one of my previous posts on this thread:
"I continue to think that the vibration is originating from around where the turbo charger connects to the intake manifold."

From the technician I took the car to today:
Of the five nuts securing the intake/exhaust manifold...
One was just a little loose.
One was finger tight.
Three took 1.5 ratchet turns each to tighten.

My mistake was *visually* checking the nuts and not sticking a ratchet onto them to be sure they were tight.

I'm certainly glad that the problem was nothing more than loose nuts.

As I long suspected and stated in the quote above, the vibration sounded like wheezing when I leaned over the turbo charger and listened to the engine when it vibrated.

Lastly, the technician said that he thought that re-tightening these five nuts is part of the 15,000 mile service.

Once you know something is on correctly... you can use this type of paint so that a visual inspection is reliable.. :::

http://www.abbyair.com/tips.htm
""15. Torque Seal Lacquer – This is a product that you can paint on bolt threads to verify that the bolt/nut setting has not moved. If a bolt or nut should loosen, the paint will crack; allowing you to know the connection has loosened. Using this sealant will greatly shorten your preflight time. A simple look will verify the integrity of the mechanical connection.""

http://www.avweb.com/news/maint/191176-1.html

http://www.aircraftspruce.com/menus/to/safetywiretools.html

http://www.aircraftspruce.com/catalog/topages/safetywirekits1.php

http://www.opcompany.com/opco2.html

The combination of safety wire AND the ( typically Yellow ) safety paint droplet at the junction of things which can loosen.... can make checking for things like this pretty fast and trustworthy even though they are ' just visual ' checks...

__________________
1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/diesel-discussion/10414-help-i-need-check-stretch.html

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html