knocking noise from the head

[335JET]

on saturday, i started my car and there was a knocking noise coming from the head, so i turned the car off. about 30min latter i started it and it was fine.
today i started it in the morning and the same knocking noise, however not as bad so i drove to work , then this afternoon i couldnt hear it.
- as i rev the engine the knocking gets faster and at idle its fairly slow.
does anyone know what this is.
also there is no performance loss.