Strange Boot Up icon

i noticed that about a 2/3 weeks after getting my MBP that when i start up, were the nice apple logo should be (while loading) there is a pic of a folder with a questionmark in it. the pic disappears after a couple of secs and the apple logo returns.
i didnt notice and changes in performance after seeing this so i guess its not doing any harm, just wondering if anyone got ideas as to why this is happening?
thanks

That icon means that the mac is searching for a startup volume. Obviously, since OSX boots up, it can find it. You can try to set startup disk preferences in the system preferences menu (system preferences ->system -> startup disk)....this might make it "unconfused".