| There are a few things you can do to ensure that these samples don't "pop." I'm also not an expert in Garage Band but every digital audio editor should work this way...
The first thing to do (and you should always be doing this no matter what) is to start and end your samples at the zero-crossing. Examine where your starting and ending selection points are being made. Take a good look at the starting and ending points of your waveforms up close. They should always start and end where the waveform naturally crosses the zero-voltage line. If you start or end a digital sample where the waveform is anywhere other than zero, the voltage of your D/A converter's output (on your sound card or I/O box) will "pop" because the D/A converter normally rests at the zero-voltage level. When the sample begins, and it immediately requires your D/A converter to jump to a drastically different voltage level, it will sound like a "pop" or "crack." A lot of pro digital editing gear will at least give you the option of always auto-correcting your sample selection points to the nearest zero-crossing point, but some of it does not.
Even when you're doing this, it may not help, because a sample may contain a loud transient at the beginning - it's just how it was recorded, and it's supposed to be there, but because the normal sound level is so quiet it may just jump out and surprise you and be rather unpleasant. So take just the first few milliseconds of your sample and fade it in, and that will help to make it sound a little more natural and less startling.
Hope this helps - I had the same problems you did once, and I had to learn the hard way. :\ | 
