I've noticed the past couple of times that I've started up my X in the morning it takes on this chugging sound. It's this faint puffing, almost like a steam engine or something. It only happens when it's cold outside, and it goes away after the X warms up. When I look at the exhaust I can see this kind of puffing action happening, nothing to serious, really faint. I'm not worried (unless I should be), but I would be interested in an engine-minded person's explaination as to why this happens.