Okay, might just be something I have come upon.
More Questions:
1. Slow speed= noise Fast, no noise?
2. Only on side?
3. Recently been on gravel or loose pebble roads?
IF so....
How handy are you? Take the wheel off and you just might find that there is a pebble stuck in the rotor and it's banging onto the brake guard. Centrifugal force keeps it from moving when you're at high speeds.
Here is more info I had conversed with a person with a similar problem:
http://www.yarisworld.com/forums/showthread.php?t=14220
If you don't think you have this situation, give us more detail, anything you can think of.