GarageBand Monitor Feedback

So here's the problem.

Here I am, ready to have some fun, make some music. I plug in my guitar (via my GuitarPort) through a MIDI-gate. Set up everything I need, then make sure I can hear what i'm playing so I turn on monitoring (feedback protected), you know the second choice, between "off", "on" or "on (feedback protection)". But then all of a sudden I get feedback. But only very short, for about 3 seconds (ok maybe, 2.5). Then there's 10 seconds of me jamming, and then the short feedback again. It's always like this, even when I turned the volume and both the tone buttons down to 0 on my guitar.

The question is: 1. Where does it come from? and 2. How can I fix it?

I hope someone understands my problem here and is able to help me out.

In any case, thanks for reading this and helping me out.