Mine was doing that...dealer looked at it while it was in for a recall repair and found that a hose had split and was sucking in fresh air. I'll try to remember to find the service report and let you know for sure, but I want to say it was a hose for either PCV or EGR, and it was something that (allegedly) ran a good ways around the engine. I got nicked $180 or so for repairing it.
