2010 Other GMC Models Rattle

I just purchased a 2010 GMC Terrain. In the beggining I had no problems but about a month after purchasing I started to notice a faint rattle coming from the passanger front side that to me sounded like a "baby rattle or marbles" and it would ONLY happen while accelerating. As soon as you let off the gas or stopped or in park it would not happen. The noise is now a lot louder to where I can hear it out of the driver window as well. Still only happens when accerating. The car dealership thought it may be something in my muffler and messed with my heat shields but I'm not sure. They plan on replacing the muffler soon. Any ideas?