AudiForums.com - NOISES from the car

Noises are one of the signs that something could be wrong. Noises even could occur a lot before something really bad happen to your drive. I am sure that most of us here are subconsciously listening to every single sound while driving and subcounsciously become aware of any sound/noise that sudden occurs no matter how silent it is. However, some people ignore these sounds until it become late. Here is my problem, that I cannot resolve by myself.

My car is Audi 100 turbo diesel, 2.0, 64kW, A/C, year 1987, FWD.

I hear noise/sound like whining somewhere from the engine (somwhat down-left I think) while I drive on the flat road. It occurs ONLY at around 65km/h, no matter the gear (3rd, 4th or 5th). I hear it most clearly at 4th gear. Point is that it happens while I am maintaining approximately constant speed around 65km/h. When I accelerate or decelerate it stops immediately. When I press the clutch pedal it stops immediately. Gearbox contains 80W-90 Liqui Moly transmission oil. What do you think people? It has nothing to do with motor and gears, but I can't realize what it could be? Why exactly at that speed?