The singing gearbox...well who knows?! You have so many bearings in there.
My sedan's gearbox is original and it sings between 45 and 55 mph. Not unbearably, but noticeably. The throwout bearing also whines like crazy whenever the clutch is operated (especially under low load).
Sometimes I get angry at it and sometimes I tell myself that it'll hold. Never has jumped out of gear, but 1st to 2nd is notchy.
Maybe things will get better after a motor mount overhaul.
You shouldn't worry about yours if the gear changes are relatively smooth and non-grindy.