twin tailpipe poping sound

car was up on ramps today having the Geometry re-done and the car had the engine running at idle for a few mins and as I was standing behind it I could hear these sort of popping sounds coming from the exhaust tailpipes , not very loud at all, but all the same they're there! almost sounds like when you have a small hole in the exhaust and air is popping thru if you see what I mean, exhaust appears to be sound and has only done about 5,500 miles from new on an 07 plate , so wasn't really expecting anything too drastic with the exhaust and even blocked off the tail pipes for a few seconds to listen for any air escaping and seems ok, maybe it's normal due to the twin tail pipes I don't know?
Cheers Pete