2001 Ford Truck noise

One of my wheels is making a mosquito sounding noise when I'm under way --it comes and goes and kind of sounds like a europeon emergency vehicle siren sometimes and otherwise like a buzzing mosquito. Brakes are good. When I drove it last and it made the noise I hit a bump and it stopped but I'm not sure it won't come back -- could it be a wheel bearing or just some dirt caught in the wheel. If dirt in wheel will it go away on it's own or should I have the wheels removed and cleaned or what? Thanks for any help. Tina