Those jockey pulleys have plain bearings, and once the grease dries out, they'll chirp like robins in the springtime.
You can temporarily quiet them by squirting some lube as close as you can get to the pulleys' centers,
or you can solve the problem for a long time to come by removing the pulleys, cleaning and re-greasing the bearing surfaces, and reassembling them.
One tiny tip: Invert the bike, remove the rear wheel, and before you do anything else, snap a cameraphone photo or three of the RD. That way you won't struggle Rubik's Cube style when you go to reinstall the pulleys and properly route the chain.
Otherwise, this is a dead-simple bit of maintenance.