Car Making a Squealing Noise While Driving?

I have a 2005 Toyota Scion xA with about 51,000 miles on it. While driving last night, the front left wheel started making a weird twittering/chirping noise and the faster the wheel turns, the more frequent the noise and it's only coming from that front left wheel. When the brakes are applied, the noise stops. I had my grandad (who's a certified mechanic) look at it and he said that it's not the brake pads and that he didn't hear any noise. I'm not sure what this could be and I'm not sure what to do.