[alloro]

On the back of each cylinder head is a freeze plug. There are also two freeze plugs on the back of the block that can only be accessed after the tranny is out. Lastly, the back of the intake gasket could simply be leaking.

Did you take the engine cover off to look for the leak on the back of the engine? If you did and could not see any leak, then it's probably one of the two hidden freeze plugs that you have to pull the tranny out to get at.