thats fine often you dont get enough screws. My motherboard isnt secured down through 3 holes so your fine.
There may be wires in the way of the fan, try moving some stuff around in the case, same thing happened to me.
what i believe it is is that there is a nick or something stuck the a fanblade, which causes it to be unbalanced and makes it buzz alot, or theres a cable hitting you blades.
Hmm yeah my computer makes a weird ticking humming noise. Its a dell. Im pretty sure its the power supply. Oh well, im to lazy to fix it, and im getting a new comp soon anyway. (Not from dell ;x)
