There is a gear assembly in the transmission. With respect to the gear assembly, one gear has teeth on the outer and inner of itself (I call it the ring gear). There is another gear (shaft gear)that syncs up with the (for lack of another term) ring gear. With that said, I believe the two gears are not fully disengaged causing the noise you are hearing.
It depends on your expertise regarding Kirby's. Anyway, dissemble the Kirby motor unit down to the transmission and see if the bracket Cam assembly is broken or just dysfunctional in some way. Here is a good video showing the actual bracket cam assembly.
