The reason why this happens is the contact points inside the air bag have worn out. If you'd like you can also fold up a business card and slide it between the air bag and the air bag frame. Its a temporary fix until you can get the air bag frame installed. I had the same issue and didn't have the money to replace the air bag frame and I'm not willing to change the air bag myself as I have heard nightmares from people who have had accidents with installing air bags. Another member told me about this...and no it does not hinder the air bag mechanism in any way, but it does solve the constant honking problem. It doesn't stick out especially since I used black tape to cover.
Boxster with the honking horn - Solution - Car Talk
hope this helps
