I've heard this on some Spotify tracks (no way will I remember which) and put it down to compression artefacts caused by the lossy conversion.
I've just checked '3 Doors Down - When You're Young' on Spotify and sure enough the artefacts are there.
I can't check iTunes without buying the track because there's no way of making it preview the first seconds, but I presume they will be there.
To quote someone who posted a comment on that website: "It is a strange thing that the real quality audio is now reserved for the pirates. This industry really knows how to hit its targets."
Indeed.
Ah well, just another valid reason to continue buying CDs. Not that I was looking for one.