What kind of vacuum pressure should I have with the engine warmed up and at idle?

I also have a popping sound coming from under the hood. Sounds like someone popping a loud bubble from chewing gum. All I have done is pull off a block off plate on the intake manifold and re-seal it with RTV. Also put on a new PCV valve.

 

Well the popping noise stopped and have about 2" of vacuum pressure at idle with engine warmed up.