Thank you for your replies. My hubby says, after I drove him around, that its a High frequency sound. And it starts for about 3 seconds, stops then starts again. Does not do it when the car is just sitting, only when being driven. Very odd!

I don't think it would be leaves, because of the piercing sound it makes. I thought that maybe playing the radio to loud once, that I wrecked a speaker, but with the radio off it still does it.