Wind noise question

I'm getting used to my 2000 Four Winds. The chassis is Econoline 450. I was driving along wondering if the wind noise I was hearing coming from the passenger's side was unavoidable, when it suddenly stopped. Like a door closing. After a while it started again just as suddenly. I was on a long trip, and as I went the whooshing started and stopped randomly, with no link I could see to anything I was doing. I have a little trouble localizing it other than it seems to be happening behind the dashboard on the passenger side.