Air-conditoning Odor

Upon entering my home you can smell a difficult to describe odor (bleachy, metallic, ozone) are some words used to try to describe it. It is definitely connected to the air-conditioning unit because it is not present unless the air-conditoning is turned on. One techinician said to put some bleach in the drain pipe. I do not know what this would have to do with it. I did, however, do it with no apparent change. ????? Thank you.