Strange smell

The A/C has been operating normaly until last week. we used it 2 weeks ago with no problems. It wasn't used for one week. The next time we went to use it there was a strange smell with the cold air. I have since removed the case and used Clorox wipes to clean the squirrel cage. I did not notice any mold. I noticed some rust stains at the bottom of the radiator (the cold one). The rust was around the bottom of both side supports for the cooling pipes. I could just faintly smell the scent when I sniffed the fins. I could also faintly smell the scent on the squirrel cage fins. I replaced the styrofoam and turned the unit on. The smell was still there, and as strong as last week. Could I have a coolant leak? What would that smell like? How can I check for a coolant leak? Is it time to call in a pro?