I have a 1987 Oldmobile Firenza and I tried to take it up a steep canyon the other day, but ever since then it's been making loud hammering noises and the oil light comes on. I got an oil change just yesterday, and they said everything was full. The oil light comes on when I am in neutral or brake and goes off when I accelerate. The car is less powerful than it was before and slower to accelerate. What's wrong?