Sounds like a mildew problem....pull front cover ..air filter ect and expose evaporarator ......has all the fins ect ....
You can go by local air condition shop and ask them for a cleaner or have seen people use a 50/50 mix of bleacjk and water and with spray bottle spray in it the unit ..don`t worry it should drain out the back drain .....if it isn`t clogged ...witch it could be and is causing problem 
in which letting this solution soak will repair problem 
Hopefully this helps
                
 bg
                
 February 2007