I have had a creative zen xtra and I now have an ipod 4th gen 40gig. On both there has been a 'crackling' in a low frequency band, i.e. during heavy bass and crashing guitars. I've tried a couple of permutations with codecs but this doesn't seem to have helped. Has anyone else experienced this? Does anybody know how to remedy this? P.S. I don't have it exceptionally loud or with boosted bass.