Soon after I purchased the car, I could hear a rhythmic hissing sound coming from under the hood. I popped the hood and noticed a plastic tube that goes into the engine was broken and there was air hissing from it. It turns out that it was the "evap purge solenoid" which takes air from the exhaust and recycles it through the engine to burn off and unburned gas. The dealer fixed it under the CPO warranty.