could be the coolant tube seals, water pump or maybe intake manifold. I'd pressure test the system and look/listen for your leak. You can get a coolant pressure tester from any auto parts/tool store just take your rad cap with you so you can get the right size adapter for your rad. Then plug in the tester to your rad and pump up the system. The max pressure should be on your rad cap. DON'T exceed the pressure on your rad cap (15-17 psi i think). Get a light and look for drips, bubbles or hear a hissing sound. The gauge will show a drop in pressure. Where ever it's leaking from is your problem.
A coolant pressure tester will run about 40-60$