If you can get a scan tool , check your fuel trims to see if indeed you have a vacuum leak. You might see a bunch of things with the scan tool anyway. Codes for vacuum controlled outputs or inputs are very helpful in narrowing the search to a system or component. Watching the o2's is a waste of time unless you have alot of diagnostic skills and alot of practice.
If you can hear it in the car, second thing you might want to do is to pop the hood and plug the line going to your heater controls. If the noise goes away you have narrowed the fault to the heater control system.
If it's still there get your hands on a stethascope. If you get a steth with any kind of attachments on the end , remove it so that you have the tubing exposed. With the engine idling slowly move the end of the tube around any gasket surfaces or components subjected to vacuum.
Use a water spritzer to spray small ammounts of water onto any suspect areas. Sometimes leaks will make alot more noise as they suck water into them.
Using carb cleaner or brake clean to listen for changes in idle speed is not a very good idea, even though alot of people try this.It's not very affective on FI cars anyway.
Some well equipped shops have smoke generating machines that pump harmless smoke into an engine (not running) to allow you to trace leaks quickly.
All in all, finding vacuum leaks on a fuel injected car is quite tricky sometimes. I can tell you that on any late model Mustang there are problems with the stock intake gaskets and people who have done alot of porting and then installed a blower. Vacuum lines if not safety wired can pop off under boost situations. Good luck finding the noise whatever it is.