One of my friends '99 Mustang is having alittle problem. It seems to be rattleing but yet the exhaust is on super tight with nothing hitting anything. It only happens low mph like backing out of the garage and back into it. Could this be a trans problem? And second off his car also is almost sputtering like around 4K if he floors it. It sputters only in that gear and as soon as it shifts its fine. ITs an auto.Thanks