pitched hiss which is the air escaping through the exhaust ports. Anything higher and that is your leak. Mixing a thick solution of soapy water to spray on your leaks will give you bubbles to help identify exactly where the leak is coming from. The only way to hold in that 5psi is if you taped up the back of the exhaust.
This is what I recall from an old search but I've only tried the first method.