Screeching sound


My Jetta makes a weird screeching noise (like metal on metal) when I take left or right turns… mostly left. For a while it was a simply squeal… and only when I turned the wheel into or out of a parking spot. Now it happens when I begin accelerating, in a straight line, from a standstill. It seems to be coming from the left driver’s side wheel but my mechanic found nothing wrong (he checked all 4 wheels). He suggested it might be the little rubber shock absorbers but I’ve heard those (they do squeak). This sound is definitely metal on metal. Could it be that my wheel bearing is broken? Wouldn’t my tire have already fallen off?