You might have mold or mildew in it. That takes bleach to clean it. Read my post under WARNING ALL CAPS about the medical problems that may arise. I thing Legionaires disease came from an a/c system.
Set it on the ground(unplugged), take the cover off, hose it off, brush it with soapy water(a bucket and brush like your car), and when done get you a quart of bleach and soak it with bleach for a half hour or so to get rid of mold and mildew. Water shouldn't hurt anything but maybe some of that black insulation when it gets wet. Avoid direct spray into the motor winding but let it dry if you do. Spray the fins in the coil from the inside to clean them out. Oil the motor bearings and fan. Don't plug in and run till completely dry.