I have a 93 tercel with the 1.5 12 valve motor. I just bought the car yesterday with 106,000 miles on it. It ran perfect until i had driven about 100 miles and I started to hear a loud knocking in the motor. It does it all the time when I give it gas but when I let off it stops. The cars performance doesnt seem to be affected. I checked the oil and it is full. What could be causing this?