Odd random clicks on iPod Touch during music playback

I have the latest generation iPod touch and have noticed that when I use it to play music through my car (via USB interface) there are random clicks and pops sounding almost like the tracks were transferred from vinyl records (for those of you old enough to remember those things. They are relatively quiet, but definitely there.

When I play the same tunes on on iPhone 4 (via the same USB cable), the songs are fine.

Any ideas?