I bought an old VW Golf II recently. The gearbox was whining when driven in 5th gear between 75 and 90 KPH then it would stop. I suspected gearbox oil, so I had it changed. I was there when the mechanic drained it out and it was totally black and had low viscosity. It was most probably used engine oil that previous owner had put in. The good news is that there were no filings. Now with the new oil (proper gearbox oil) the whining is not as loud, but it can be heard from 70 KPH to over 100 (haven't driven it faster). It disappears when I lift of the throttle or press the clutch. The volume of the sound changes with the amount of engine load. No other gears make this noise.
So, it's probably a bearing, it might be under side (axial) load. I know I'm looking at transmission rebuild some time in future, but should I be worried about this right now. It's not very loud. Should I be avoiding loading it in 5th gear, that is, should I be going down to 4th for hills and overtaking?
If it's a bearing it should become louder if it heats up during a long trip, right?
What is the worst (realistic) case scenario?