Carb vaccum leak?

My 71 Dart 318 is making a "tea kettle on the stove" noise when it is warm at operating temperature. It has a Weiand Stealth intake and a Carter AFB 625 CFM carb. Last Sunday I took this cap off of a vaccum port on the carb down by the choke housing. With this cap removed the noise stops. If you put your thumb over the port the noise starts again.

I am thinking I have a vacuum leak somewhere on the carb and when this cap is removed it changes the flow enough to make the noise stop? Could this be the gasket between the choke housing and carb body? Any help would be appreciated.