I'm not sure if anyone has seen this link, but it confirms what I have thought whilst listening to certain Spotify tracks:
Basically, some music providers encode a water mark into the tracks that are in fact audible to the ear.
They sound, when in a track, rather like they are lower quality tracks, because of the weird pulsing you get.
Has anyone encountered this?
Seeing as most Spotify tracks are encoded at 320kbps when in Premium mode, I was surprised to hear weird artifacting on quite a few new tracks, and upon searching, found that link.