Q: Replaced alternator and now there's a low sounding, high pitched noise, like a jet engine whistling.

Hi and thanks for contacting YourMechanic. This can be an easy fix. The alternator may have a problem with the fan blades inside it if you got a brand new design. If you got a design with the fan located on the outside of the alternator, then the fan is too close to the vent slots and is causing turbulence within the alternator. Look at the drive belt and see if the belt is straight on the pulley to the alternator or if the pulley needs to be pulled out a little.

If the pulley needs to come out some, you can put a spacer behind the pulley and fan to bring out the fan, which will help the fan blades from producing the turbulence. If the fan is not the problem, then it may be the belt on the pulley causing the same sound. If you need assistance troubleshooting this sound, then seek out a local professional to inspect the noise and diagnose the loss of power for you.