Recently there's been a strange noise coming from the exhaust, the sound isn't there when driving but shows up at low idle. It sounds like "bub, bub, bub, bub..." but only on low idle. I don't feel any loss of power and the scangauge isn't showing any codes or overtemp. It seems to be a little worse in the cold weather. Any ideas? Stock engine & exhaust but does have a CAI. I first noticed this in the fall but seems to be happening more often. I'd like some ideas before I run to the dealer, wrong time of year for major car repairs, maybe covered by extended warranty?