Just a few days ago my Automatic 97' started to make a loud Lawnmower-like sound out of the Muffler when revved. It only makes the noise and shakes as as if it is about to die when in Idle and First gear. The noise and shaking stops after entering second gear. What is the problem? The only thing we did to it recently was change the valve cover gasket, and put different oil in it... cheap oil, if that makes a difference, and change tire pressure. Thanks.