Serious problem with garage band plz help

Ok if im in garageband and im making just a basic track, in the track info if i select monitor on i geta krazy feedback the macbook actually vibrates, *** is going on does anyone else get this when they switch on monitor?

What does monitor even do?

Monitor plays whatever is being input to your machine. If you're connecting a guitar for instance, it will play through your speakers. My guess is that you are running a voice track and it is feeding back. Either cut back the input volume on the voice track (if you need one), or use headphones for monitoring.