Thank you Garret and Trevor. Has the filter has been changed. As per the service dept. at the Audi dealership I go to, there is typically a residual fluid when a/c is used. This area is hard to reach and its recommended that a professional should clean it up. The other solution they recommended was to leave the fan running without a/c for about 5 minutes before i turn off the engine.
I will try the smoke grenade type cleaners if that helps.