Internal Microphone

I don't think there's anything you can do about that.

the internal mic is an omnidirectional mic, and the clicking you're hearing is probably the sound that the hard disk makes as it's writing the files you're recording. If you record to an external hard disk, you might find that you get less clicking.

The clicking noise is probably the metronome in GarageBand. Try this: click on the "Control" menu and then make sure that "Metronome" is unchecked. if it is checked, GarageBand will emit a clicking noise to keep the time if you were recording a song. This clicking noise would be picked up by the internal microphone when recording.

If metronome is already unchecked, it might just be background noise... make sure you are recording in a quiet environment for the best results.