whining manual gearbox

[ToothbrushMan]

Put a secondhand gearbox in my car to replace my old one that had lost reverse.
Its a 5 speed manual. Old skool so no dual mass.
Anyway the new box works fine but whines terribly in all gears if youre on the gas pedal.
As soon as you take your foot off the whining stops.
Am I right in thinking its not the diff bearings because if it was it would whine all the time even in neutral?