5 speed - whines in 1st-4th
 

My '90 w201 has a whine in first, second third and fourth gears, which instantly vanishes in fifth. What's the beef?
It's the getrag five speed gearbox as fitted to the 16v, where fifth is a 1:1 ratio. This makes me assume the problem is in the transfer gears that lower the ratio in 1st - 4th since the whine vanishes in fifth? Sorry I don't know much about gearboxes. The gearbox already had a rebuild, first thing I did when I bought it, but the whine remains :(.

Could the problem be related to diff installation, prop shaft problems, etc? I ask because the car's diff was replaced with a used one by the previous owner, this was done at a garage that I do not have faith in. Could they have installed diff wrong to cause this problem? I heard it's possible to install the w124 diff with a shim being used as a washer - how do I check for this?

Also the car vibrates at 80+ mph, the vibration comes and goes, in-out-in-out forever until you lower your speed.

Thanks Russell

One of the 2 bearings on the cluster gear is bad.
Those bearings are loaded in every gear but 5th.

Thanks. So the £400 "rebuild" by transmission specialists was no good? It made no difference to the whining sound. It was my dad that sent them the gearbox for me and handled it all - perhaps they screwed us or perhaps he told them not to bother with it :(.
I'll see what I can get out of him.