oh and Garageband appears to have a few high pass filters and low pass filters, and a parametric EQ. You use a high pass filter to get rid of any bass frequencies you don't want - rumble etc, you use a low pass filter to get rid of any high frequencies you don't want - hiss etc. I suspect there'll be blunt instruments. But it's worth your while looking to see what EQ facilities Garageband has.

Pip, if you like you could send me what you've recorded so far and I'll have a go at EQing them. I need the practice! Send me a PM if that appeals .