Could be plenty of things:
1.) As buwish said, a stuck valve.
2.) Pre-detonation (esp. if your car has a turbo or supercharger). This means that the gas vapor (because of bad ignition timing, bad AFR, low-octane fuel, etc) is detonating before the cylinder hits top dead center. This is VERY bad for your engine and can cause a rod to go through the block if you push it.
3.) No oil (although if you have hydraulic lifters this would not matter).
Solutions:
1.) CHECK YOUR OIL
2.) Run premium fuel
3.) Get the valves replaced
4.) Call an exorcist
And buwish, I resent that statement.