Hissing sound at idle when hot? Why?
Redfish:
Knowing what in the heck you're working on would help.
AKA “66 Dart Convertible” and photo of his freshly restored car are two clues, third clue being question posted in /6 section of website.
Look here.
Syed, I also think you have a vacuum leak related to thermal expansion were hiss diminishes with throttle opening, increased rpm, and a choppy idle.
Hose to ear method may work if air stream around intake from fan is not too great, where you are listening for a hiss. Spraying water may be your best bet, starting fluid second as these methods won’t damage your freshly painted engine parts, as possibly carburetor cleaner will.
One more place to listen to would be that remote oil filter and its related plumbing.