This is my first post to the Pelican Forum and I hope not to cause any repetitive effort.
In a roundabout fashion, I have come into possession of a 1979 300TD with 91K miles. This is my first MB as well as my first diesel so I have a little bit of a learning curve.
When I got the vehicle (for free by the way) it did not run and had been sitting for 3 years. The engine turned over but would not start and was covered in what I later discovered to be a mixture of fuel, coolant, oil, and hydraulic oil from the SLS and dirt. All leaks and drips have been corrected..
I have done the following mechanical work: repaired a fuel leak at the injector pump and fuel filter location; repaired / replaced the SLS hydraulic pump and lines, replaced the radiator and A/C condensing coil, dryer, associated lines to the coil, and compressor; repaired water pump with new gasket and thermostat; new coolant hoses and clamps; replaced the valve cover gasket and set tappet clearance; fixed vacuum hose leaks and connections in the engine compartment; replaced coolant and glow plug temp sensors; repaired the master cylinder and brake fluid reservoir; put on new pads and rotors and bleed the brake system; flushed and replaced the hydraulic fluid for the SLS so it now works. I have changed all the filters and fluids. The engine now starts right up and from what I know and can tell runs pretty smooth. I did a purge of the system to clean the injectors. I did a compression test and got 315 to 330 psi on all five cylinders.
SO what is my problem you ask? Well the transmission is making a whining sound similar to what you would hear from misaligned gears or from a tire rubbing against some metal. From my test drive, I have determined the noise level varies with the speed of the car and gets loud enough at 35+ mph that it drowns out the engine. The trans shifts (up and down) smoothly but I do not think I am getting into 4th. Maybe 45 mph is not fast enough. I do not want to drive the car until I figure out the noise and make repairs. With the car stopped and parked on level ground I cycled through the gears and all worked fine. The noise is present when I rev the engine in both park and neutral. I cannot find any indication of any vacuum lines going to the trans. There is a single electrical connection on the passenger side. I have adjusted the linkage thinking that perhaps it was out just enough to keep the trans from setting correctly into the various positions. This made no difference. The trans fluid I am using is Valvoline Max Life High Mileage Dex/Merc, Mercon LV, ATF and I am at proper operating levels according to the dip stick when I checked after my test drive with the tranny warm, parked on level ground and engine running. Any Suggestions as to what to do next would be greatly appreciated. I am really hoping it is not internal to the tranny.

5/10/17. I have discovered that my trans is a 722-118. From what I can find it appears there is no vac modulator valve for the tranny. The only connections to the tranny are two rods, one from the shifter and one from the throttle lineage. Thee is also an electrical connection from the knickdown switch under the throttle pedal.