I have a 2009 Kia Rio with ~75,000 miles. After I first start the car and accelerate away from where it was parked (especially if I accelerate hard), the car will repeatedly make a quick sound that almost sounds like air escaping out of a high-pressure valve or something. Maybe every 10 or 15 seconds there's just a quick burst of "pfft." By 5 or 6 minutes into the trip, it seems to work itself out. I've noticed the sound seems slightly more common during gear changes, but is not exclusively related to shifting.
Edit: Other details...
- Started relatively recently (2 or 3 months ago)
- Seems to be getting worse - originally, it would do it once or twice during the early minutes of my trips, and it wouldn't do it every time. Now, it does it 10 or 20 times, and it's nearly every trip.
- Gas mileage seems to have been reduced, though it's hard to tell w/ summer fuel blends, A/C usage, and the like.
Any ideas what this could be? I thought misfiring was a possibility, but the check engine light isn't coming on. Is this something that's potentially a big enough issue to get checked out?