rebuilt the head. If the leak is large it can prevent the car from starting. Say if the plenum gasket was not seated properly. Also insulators at the bottom of the fuel injectors can leak air. Doesn't happen often, but can happen, especially if the injectors were replaced and new insulators weren't used.
If it is leaking from those areas you may be able to feel it during a vacuum leak test but you won't hear it because the air is escaping too fast to make a sound.