Might simply be the heads parking? Check your power saving options if HDDs are set to spin down after a period of idleness. Disable it, and then listen for the noise to appear again.
Temporarily unplug the burner to eliminate it as a source of the noise...
Also, if you have a SMART utility, look for seek errors. HDDs might click when re-calibrating their heads, if they feel they hit the wrong track a lot. That might show up in the SMART statistics, although it would seem these figures are often unreliable (alledgedly due to manufacturers not wanting to advertise flaws with their product...
)Feel free to quit the Steam client anytime it makes a bother of itself; it will quietly resume download without a fuss next time you start it. Or you could disable auto-updating for the game, but you'd have to do that manually for every Steam game I think, I believe there is no global option in the config menus - which is a bit of a bummer if you ever are on say, cellular internet connection, and suddenly Steam decides to start pulling down a humongous patch, eating up all your dataz for that month...
