MJohnston

Yes it makes the noise the whole time when I'm in 4th gear,   It's not a humming noise though, it sounds more chattering,   Like you say potentially chewed up gears. 

The unit seems to work great except 4th gear.  When I use the foot shifter to go into 4th gear the unit makes chattering noise.  Almost as if some teeth are missing on a gear. Even though it makes this noise, it still seems to run ok in 4th.  However I always shift right away into 5th gear which works just fine.  Has anybody experienced this and found the cause and solution?