trans leaking again after rebuild

If it a little over filled it can vent the fluid until the level gets down a little more.

Sometimes clutch discs in the trans can "bark" on the shifts (it would make the noise right as a gear takes hold.)
I put a Barnett HD racing clutch in my bike some time ago and it does that. (barks on the engage and sounds almost like a tire bark)

A whistle sound could be an intake leak at the carb base so look for that also.