Safely raise both drive wheels off the ground, and then you will be able to observe it while it's operating. Have a few cans of brake cleaner on hand. Before you running it spray the entire area down with brake cleaner. Only doing it when it's hot makes me think gasket, o-ring, or seal. There are only a few places it can be coming from. They are valve body, valve body gasket, oil filter, trans coolant hose, trans coolant nipple ( there is an o-ring on it). Worse case scenario would be a cracked housing. Good luck, if I think of anything else I will let you know