Sounds to me like you have an exhaust leak in your engine bay. The ventilation intake is under the passenger side cowling, so when you aren't moving the exhaust accumulates and is sucked in by your vent. I'd check to make sure all of your exhaust tubing is intact and that your exhaust bolts are securely fastened.
What's that orange car parked outside of Buckingham? Oh yeah that's right, it's a Focus.