You may have a leaky head gasket behind engine It sometime can be visible. If it actually is leaking from between engine block & transmission bell housing you may have a leaking soft/frost plug located in between transmission bell housing/block or a cracked block although I've never heard of a cracked block in this area, I would strongly suspect a leaky or blown soft/frost plug which would be located in between block & bell housing. You would have to separate both to repair, this is a pretty easy repair, the hard part is separating block & bell housing.