1999 Diamante loud ticking sound

My diamante has been making a loud ticking sound that seems to be coming from inside the dashboard somewhere behind the stereo and the air con area. The only way it will stop is if i have the air on and when the tapping sound stops i can turn of the air and it wont start again unless i turn the car off. I would like to troubleshoot this before it gets worse i cant afford to take it to a shop. Ramon