On the weekend, my wife comes to me and says her car is making a knocking sound. She's worried something is wrong with her brakes and wants me to take it for a spin. I oblidge, and sure enough I hear the knock too... only happens when braking fairly hard, but doesn't sound like it is related to the brakes per se. I tell her it sounds like something is loose underneath, but when I crawled under I couldn't see anything with my untrained eye. I tell her to take it in to Toyota for them to have a look, and she does. For the record, it took a second mechanic to diagnose it... their laughter was payment rendered.