No fluid added when I returned, as then, the mechanic said the fluid level was fine. I believe, though I cannot remember exactly, if the engine was warmed up at the time he measured it.

Since then, had new transmission fluid change, gasket and filter. Also had to put new transmission pressure modulator. When my mechanic replaced the modulator, I realized that there might be a lot of 'varnish'/buildup in the trans....so I put in a bottle of trans x. Since then, no change.

This morning was the first time the noise/vibration started at 35 mph. Because the rpm was low, it may have been in third gear, but I am not 100% sure.

This might narrow the scope to being 3rd and 4th gear specific. Not sure what that could mean- transmission related? If so, any thoughts on what trans issues could cause this?

My first guess would be that it is related to the transmission as it goes away after you put the car in Neutral. However there are other factors at play. The engine being under load at a specific RPM. As was suggested timing, injectors, and poorly adjusted valves (compression?) could also combine for some strange harmonics.
I'd start with the motor. You can test the above without throwing unnecessary parts at the car and they are all just prudent preventative maintenance. With the motor ruled out you can start to look at the transmission or other possibilities.

You didn't mention front wheel bearings. Were they replaced? And what did you mean by "axle bearings?"

I'd start with the motor as was already suggested. You can check the timing, valve adjustement, and injectors before throwing more money at the rest of the drive train. Those things are just good PM anyhow

Good suggestions. Because my mechanic had already adjusted the valves in the same job as he did with replacing the rear axle shafts, we did not think to look there again.

How would you test for timing and fuel injection? What types of noises have you noticed in the past with these issues?/vibrations

I would disconnect the speedometer cable at the transmission and see if the noise goes away. Your description does not have the classic symptoms of a speedometer or cable problem, but it does match some of the symptoms and it is an easy check.

__________________
1975 240d/4 Speed "Old Blue"
1985 300d/4 Speed "DaWhite"
2000 Toyota Tacoma Prerunner

front wheel bearings were not replaced, as they did not display the typical symptoms after rocking/shifting/turning the wheel while the car was on the rack.

Axle bearings= please forgive wrong terminology, but there were some type of bearings associated with the installation of the rear axle shafts.

Injectors have to he tested with a pop tester and timing....well search the forum this has all been written up before. As for noises poor timing and bad injectors can sound all sorts of funky when the engine is under load.
Sounds are hard to diagnose without hearing them. I suspect you have a transmission gremlin but the metal in your diff is suspect as well.
I have read somewhere on here that when looking for these noises start at the engine and go back.

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UPDATE

Earlier this week and today, noise/vibration ocured for the first time below 40mph. Was in 3rd gear with low rpms around 30mph on highway.

I think it is time to make a video with sound of this vibration.

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

UPDATE- Problem (kind of) fixed.....

Recently had the privilege of having a highly qualified/experienced Benz mechanic review the car. Very quickly, he saw that the center support-portion of the transmission mount was positioned in a way that rendered the mount useless; therefore, it was as if my transmission was sitting directly on the car. After removing the center portion of the mount, the vibration disappeared. At that point I was satisfied. The whirring noise is still there somewhat in the same speed sections.

I know there is still a problem. However, I am enjoying the moments whereI at least have a vibration-free car...finally.