are all your hard drives set to the highest UDMA mode possible? i used to have this problem too. at first, i didn't know what was causing it, but i found out that whenever the hard drive performed a read/write or request, the sound would skip... i later checked and it was because my drives were set to PIO mode instead of UDMA mode 5 ... i changed it, and no more clicks, pops ,or skips
EDIT: oh wait, nevermind, sorry didn't read the SCSI part =P