I'm 99% certain the noise is coming off one or both of the derailleur pulleys, but not sure if it's to be expected or if it's irregular and something that should be fixed?
Edit
Just remembered - I tried removing the back wheel (including the cassette, obviously) and the noise persisted, so it's 100% certain coming from the pulleys. I just can't determine why, but I wouldn't be suprised if that's how it's supposed to sound.