whistling noise could be an intake leak, like loose bolts or bad gasket lettting air in? hmm come to think of it it could be normal, my friends 3sge makes a whistling air sucking noise around the intake when you pull the thottle cable abit, of course the engine noise drowns the sound out the faster you rev it.
Ok that screw looks like the Air conditioner idle up screw. Turn your air con on, and turn that screw and see if the engine revs change.