Gearbox has a whine to it, which i never liked at all so i put a noisier end can on to drown it out.
But you said you haven't run the bike since, so it may have been the chain slopping about.
You need to try it then see if it's improved, but i must stress again, do not have the chain too tight, or you'll have more issues.
Do you have the new front sprocket nut on?
I'd give that a check to see if that's still sitting there happily as well.