2001 Grand Am 2.4 - Overheating - Coolant Disappearing - No Coolant In Oil?

2001 Grand Am, 2.4L 4 Cylinder.

Overheating and coolant is "disappearing", no visible leak's, going through a half gallon every few day's at least, no coolant in the oil, smell's like coolant at times, checked the tailpipe, it's clean.

Of course I first thought head gasket but like I said the oil is good, any idea's?

Thanks!