tranny rebuilt 9 months ago.It has developed a leak

I have a 2001 monte carlo ss that has had the tranny rebuilt 9 months ago.It has developed a leak when you start it up by the flywheel cover.Only leaks in Park before putting it in drive on initial start up when it is not warmed up,otherwise you can run it and put it in park and it does not leak.

I have started it and ran it through the gears and then put it back in park and let the car warm up and it does not leak,otherwise if you start it up to warm it up it will leak bad until you put it in gear and take off.
It has done this for the last month and is leaking by the flywheel/torque converter cover.