2008 Chevrolet HHR Engine or muffler noise?

The engine sounds like its struggling when accelerating from a start or accelerating hard from slower to higher speeds (like on the high way), it makes a sand papery air puffing like sound after the gears shift. I do not feel any abnormal vibrations or loss of power. No engine lights or loss of gas mileage. I do not hear the sound when in park and revving the engine with the hood up. I do not hear the noise at cruising speeds. It's worse when the engine is cold but still there after driving for 30 miles.