As HPL suggested, have the system pressure checked. They will remove the rad cap put on another cap that alows them to pressurize the system to find the leak.Or you can buy your own and DIYhttp://
www.amazon.com/Stant-12...SRGWUThat was just one a a few from a quick google search.Tough to pinpoint, and guessing is just that guessing.Could be something as simple as a lose hose, to something major as a waterpump or head gasket.Are you noticing a low level in just the overflow tank? or in the rad as well?No puddles?
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