1996 Chevy Cavalier What may be causing theis sound?

Over the last 2 weeks I have noticed a whistling noise coming from what sounds like the wheels, I only hear it when I am driving, so I dont think its my breaks, when I speed up it speeds up, when i slow to just about a crawl, it does also, and I have noticed it is getting louder as the days go by. I hada mechanic look at it thinking brakes, but they told me it wasnt the brakes but a tire that had lost its round shape. I have replaced the tire and still having this problem.