If it gets louder the more you put your foot down (especially at lower revs) and sounds like a very loud ticking sound (depending upon the size of the crack/break), then it does sound like it could be a cracked header or exhaust leak very close to the head (very common on a pinto).... Get a piece of vacum hose and put one end to your ear and run the other end round the extractors when the car is idling - to find where it is broken. You will hear it with the hose, even if you cannot normally when it is idling)
If it is worse at low revs but goes away at higher revs, could be a (slightly) bent valve....
It is remains throughout the rev range but just transitions from a ticking to a whiring gear type noise (not a good simalie!) then it may be a rocker has come loose or has bad adjustment (worse with bigger cams normally, but not too much that can be done in some situations)....
If the noise suddenly stop and it feels like you are running on 3, then a rocker may have been being beaten to death and either broken in half or fallen off it posts (in pain)....
Unfortunately I have had all other than the last happen to me (I do actually adjust my tappets before the rocker gets beaten to death - but a mate who has no mechanical sympathy did it - and he was a mechanical engineer. He only pulled the rocker cover off to see why it was no longer making a noise!!!)