Some mechanics shove a clean oil filter into the air hose from the filter box, to make the vacuum whistle louder and easier to find. Don't assume it will be on the top side of a fitting, you can take a wet rag and surround a fitting to see if it changes, etc.

 

A few more things to add that I have noticed recently, but may not relate to the sound:
1. Rough, low idle after I start the engine when warm
2. Stalls after revving when warm
With this it leads me to think its a vacuum leak, but I'm not finding anything. I have also found that it does make the sound when revving in park (slurping sound, not whistle), it appears to come from the area between the throttle body and the oil cap.

 

there is a plastic line that runs from that area to a plastic "valve" on the pass side wheel well is that line cracked?

you could spray the area while the engine is running with something flamable like brake or throttle cleaner to find the leak if is is vacume

 

Did a little work last night, found a way to pressurize the system with my bike pump. Put a few pounds of pressure in it and found that the idle air control valve was leaking. I replaced the gasket and it held air, but a part of me thinks that it didn't solve the problem. I'll drive it today and see what happens. If it doesn't work, I'm clueless as it wasn't really leaking elsewhere.