That noise brings back bad memory's ha. My car made a similar noise and it ended up being my oil pump committing a slow and painful suicidal death. . .
http://mitsustyle.com/forums/showthread.php?t=26546 the speed or hum of the noise increased as RPM's increased as well. If the noise doesn't clear, i would try and spin your oil pump pulley as the last option, see if it perhaps is that. It could be something loose like you mentioned as well.