It's a tight fit back there, but you just have to keep looking for it until you find it. It's probably the water pump. If it's leaking more towards the intake manifold, you could have a problem with the water pipe or the O-rings on it. Either way, the pump has to be removed if that's it... so if it's the water pump, might as well put a new one on, and a new timing belt while you're in there. The parts are cheap.