screeching is more than likely the belt..
the idler pulley(which I think it what you mean by deflector) would be more of a grinding/groaning noise.
the tensioner could be gone(check the belt tension...should have a bit of give) but more than likely you need a new belt(s)...
if its making a constant noise than its one of the pulleys. belts usually squeal(slip) as you hit the throttle.