I've noticed for the last week or so that whenever I accelerate, there is a sound coming from under the hood. It sounds kinda like air escaping. Its not as crisp as it used to sound and there is a little rattle. Anyone know what it could be?

 

i hear the same thing except it doesnt rattle and i dont think it's bad u can hear it on mine when u kinda hit the gas and when u let go of it kinda sounds like a lil blow off valve

 

^cuz it is

 

Slight rattle upon higher rpms could be damaged cat. I have the same thing because i went over a bump incorrectly and damaged it. More than likely i believe this may be the case for you.
Not a real big issue honestly - you can either go back to the dealership and get it replaced or you can go aftermarket like myself.