1999 Chevy Astro coolant leak

I have a 99 astro and there is alot of coolant leaking from under the transmisiion or I assume it is coolant because the coolant tank was empty then I filled it and it was empty an hour later but the van runs fine the engine temp is about 140-160 and I can not hear any boiling or wierd sounds and the van runs fine what could this be?