There are several potential causes for this. If the clicks and pops always appear at the same location within the same samples, it may be that the data on your drive is corrupt. If the noises are not always in the same place every time, it could be caused by defective memory or possibly a bad SCSI connection or SCSI cable. Try swapping SCSI cables to see if it helps (you may need to try a couple of different cables to be sure).