Hissing sound at idle when hot? Why?

During the run yesterday, I notice a distinct hissing sound seem to be coming from the carb/manifold area. This only happens once the engine is fully warmed-up.

I cant pin point exactly where, just wondering what are the possibilities? Vacuum hoses are all new. Carb gasket and o-rings are new. Air cleaner is tight and does not wobble/move.

The engine drives fine, but idles a bit erratic once the hissing sound started. The hiss goes away as soon as I hit the gas...

Any idea where i should start?

Thanks.

Syed