My guess is a bent rotor or the caliper is not alligned properly.
Originally Posted by
MudPie
...clean the rotor with rubbing alcohol. ...
The reason that MudPie suggested this is because if rotors get oily or contaminated they can howl when you apply the brakes. Cleaning with alcohol helps but if your pad are also contaminated the howling will continue. Incidently they will still work when contaminated so if you can live with the noise great