Noise while driving

2002 Ford Focus LX, 75000miles, automatic transmission. I bought the car at 65000 miles from a dealer. It has been doing fine since I had it 2 years ago. Had rear brake replaced half year ago. The car started to make some noise while driving recently. The noise sounds like "ta-ta-ta-ta-" and the rhythem speeds up/slows down as the car speeds up/slows down. I guess maybe 8 times per seconds at 30mph and twice per second at 5 mph. And it doesn't disappear until the car fully stops. The sound isn't loud enough to be heard when the windows are closed but is noticable when the windows are rolled down. Does anyone know what might be wrong? I'm guessing brake or shaft bearing but I'm not sure.