I have recently purchased a 2013 Hyundai elantra. So far so good but there's one thing I can't seem to figure out and it's every time I accelerate and only when I accelerate, the car makes a hissing noise, kind of like air coming from a spray can. If I roll down the passenger window, I can hear the high pitched noise and it almost seems like it's coming from the exhaust. I've taken it to the dealer and everything checked out, no leaks, no problems. In fact, a mechanic said he noticed the GLS models from has early as 2011 seem to make that noise. What causes this sound?