Sorry if there is a garageband forum already.

Anyways, when recording in garageband, after I get done playing a riff and an rest occurs in my music, there is a... "sweep" or some type of noise after I play. This isn't from my amp, so I'm guessing it's the software of sorts.

I'd really like to remove this annoyance. Suggestions?
Not familiar with GarageBand... but I think you might need to be more specific with what you mean by "sweep"... it might help to list your soundcard, amp, effects, and whether you're running mic'd or direct as well. Could be an artifact of software or a cheap analog/digital converter... or... built-in effects you're unaware of like noise gate closing or a compressor pumping.
Well you gave me a ton of options to look into! I'll be sure to check into all of them.

I'll try to go into a little more detail...

Headphone jack of my amp to the line in of the iMac. (no distortion)
I have some distortion turned on, plus a noise gate and a compressor through the Garageband.
After I stop playing, immediately there is a... noise of some sort. One of my friends has referred to it as a "quack". It's really short and it sounds like the volume is going up to a certain spot or readjusting itself?

Dunno if that helps.