Help Please!!!

[1bad02GT]

Hey everybody. I just bought a 02 mustang gt 5spd about 3 weeks ago. The car has 26k on it and is really clean.Theres one problem though. I was about to pull it out of the garage today and at idle i can hear a very light pop or puff coming from the exaust. (but enought to know that it's not normal.) I did the old put a piece of card board against the exaust tips to see if it would suck it in and it did about every 3-4 seconds. Now with my expieriences thats always been an exaust valve sticking or a broken valve sping???????I highly doubt it's a vavle spring do to the fact that it runs really good. I was wondering if anybody out there has any ideas or has had this problem???????? If so please post back and let me know. I would really appricate ideas. Thanks mike