Coolant draining way too quickly on '99 Silhouette!!

Over the past year the low coolant light on our '99 Oldsmobile Silhouette has been coming on quite frequently. I would just fill the reservoir up every so often and it would be fine.

Today however, the van began running hot, and upon checking the coolant reservoir I found it bone dry. I filled it up again, and ran the van, and it seemed fine.

Then I found out that my daughter had actually been running the heat more than the AC lately, so I ran the heat on full and the coolant started to drain rapidly. There are no external leaks, and nothing that appears wrong from the outside. The exhaust is normal, the oil comes out clean, there are no noticeably cracked or broken caps. Also, the hot air comes out fine, and the cold air comes out like it normally does, (the AC has never been very good, the AC system had been worked on/fixed before we owned the vehicle).

Any help would be greatly appreciated! I'm hoping to do this myself and avoid a trip to the repair shop, or at least to get an idea of what's wrong with it. Thank you!

With no exterior leaks, that means the coolant is going internal. Check the oil for a milky appearance. You are probably looking at an intake gasket and probably head gaskets.

On my'02, the rear head cracked and cam bearings were shot at 141,000 so I'm finishing up an engine replacement.