2004 Chevy Impala anti freeze leak

On the passenger side on the back side of the rad it is wet from about an inch down from the rad cap. I can't see where it is leaking from but the anti-freeze reservoir level has dropped again. So do you have any ideas where this is coming from. The rad cap is tight and as mentioned is is dry around the rad cap area but damp on the back side of the rad from about an inch down below rad cap to the bottom of the rad on the backside of the rad. Thanks for any replies. Frank