I still have no clue what ya mean in your description.
I'm a professional automotive tech and we in the trade describe problems with a degree of technical accuracy.
When you say it's whining in sixth gear, is it the tacho whining or is it the engine? Or... is it the girlfriend/wife on the back?
As said by Bobby914, you need to verify the secondary drive ratio (front and rear sprockets) is standard or has been altered to 'gear it down'.
If the sprockets are standard gearing, as said by CBXRider it may very well be a slipping clutch.
Or.. if it's the tacho whining chuck it away and get a new one... Same goes for the girlfriend/wife!