whistling sound in my Explorer

I have a 2002 Ford Explorer. It has developed this annoying habit of whistling when I am on the expressway. The only thing I have been able to determine is I believe that it is definitely catching air somewhere. It usually happens over 60 mph, but can happen at lower speeds if it is windy outside. Has anyone ever heard of this? I have opened & closed vents, windows, the hood, but nothing seems to help.