Check the harmonic balancer. When the rubber breaks on them, they hammer like... a hammer. You would swear the motor was gone. All it is, is the inside banging on the outside. You can't see it till you get it off, but you can check it by pulling on the belt . If you can make the bottom pulley turn a little bit, your balancer has given out. Another thing you can do is take the belt off and try it.

Sounds like the bottom end is on the way out(sound is like a thud rather than a tapping noise)noises are the most difficult to diagnose but this is my best guess....

Try a higher grade of gas