Cannondaleriders

My Cannondale has developed a horrible whine while starting
As soon as I let off button it goes away. I have removed starter and cleaned it up internally but has not helped. What else could it be? Bike runs great.

I would at least pull out the starter clutch pack, right-side cover 3 bolts, and see if you get the same sound... maybe a bit easier than pulling off the starter and having to ground it to test it.
You might find it whines under load only, or it's the clutch pack that is slipping to make the noise.