iMac G5 sound pops and crackles!!!

Hi everyone, ok, ive just about had it with this problem, im going crazy. Ive conducted numerous test to eliminate all possibilites but ive come to the conclusion that it must be the sound output from the Mac (not intel). Basically I have a pair of KEF Q 2's and a pretty reliable Naim amp which have served me well for the last 4 years or so, then i started noticing crackling sounds when playing music from itunes, they would occur at the same moments everytime upon playin the tunes. So i took my Speakers to the HI-FI shop and they tested them out with Frequency sweeps ect...no problem, then i put a song on my ipod that i knew crackled from the mac and no crackles to be heard! so it must be the mac, which is extremely annoying. Has anyone else got the same problem? please anyone, any info would be greatly appreciated. Do you think a new sound card would fix it?