I have a 2 story colonial with basement. In the basement is a 3 zone FHA furnace (first, second and future basement zone). I recently cut a hole in the door from the first floor to the basement for a cat door. Now when the furnace turns on I notice a "breeze" of air comming from the cat door. My first thought was leaks in the ductwork causing the basement to be preasurized. I went around the ductwork put have not been able to find anything of substance.

Any ideas what would be causing the "breaze", if leaking ductwork is suspected, any ideas on how to easily locate so I can seal with metal tape or duct mastic? P.S. ductowork in basement is insulated.