There are a few things that can cause this.
One is a belt or pulley.
The other two would be air being forced through a small crack. There are two ways this happens
1) a failed crankcase vent valve which causes the engine to pressureize. When this noise is happening next time at idle, remove the oil cap and see if the noise stops, or if there is a high vacuum / suction holding the oil cap down. If either occurs (noise stops or high vacuum) then it is the crankcase vent valve.
2) a vacuum leak elsewhere - leaky gasket, crack in the intake boot, etc.
If I had to guess without being hands on, I would say crankcase vent valve.
I hope that is helpful, please let me know if you have further questions, or if I can clarify anything for you.
It has been my pleasure to help today and I wish you the best of luck with the vehicle.