That's just the way it sounds.
Just enjoy your increased horsepower, torque and fuel economy!
FWIW I just put a BMW M Performance exhaust system on mine and I LOVE the way the car sounds now. It's not "boy racer" loud, it sounds just right. There are lots of video clips on line of different systems you can check out. Then you wont notice the unpleasant idling sound it makes.
