I was in the hatch area of my xj the other day, and it was idling, and i noticed every 10 or so seconds i would get a noticeable puff sound from my exhaust. The engine didnt miss at all, just a big puff from the exhaust, and it happens every 10 or 15 seconds. I thought the engine was missing, but it wasnt so i am stumped here. Any help is great. Thanks.