If you hear it out of the intake, oil filler then it is either a bad intake valve- If you have a bad intake gasket check around the perimiter of the lower intake as well by squirting some soapy water and looking for bubbles or hissing noise on the cylinder you are doing the test on
If you hear it out of an adjacent cylinder spark plug hole or see bubbles in the radiator then most likely a HG or cracked block
If you hear it whistling out the tailpipe then most likely exhaust valve
No air heard leaking but you see a leakdown of 20% or more- bad rings,cylinders worn
Normal to see up to 15% leakdown on a used street motor. What you dont want to see is a huge variance between cylinders.