An AC arc will make a noise at the frequency you have set. That is normal.
That is just how aluminum welds. It takes a hell of a lot more skill to weld than steel. What you have is an oxide layer over the puddle. The problem is that aluminum oxide melts at 3,762°F, while the melting point of the aluminum underneath is only 1,221°F. For starters, be sure to remove as much of the oxides as you can, either by grinding or brushing with a stainless wire brush. New oxides will form almost immediately, but it will be a thin layer. The cleaning action of the AC arc will help to punch through those oxides, but you will not see the kind of puddle that you do with steel. Often you can help things get started when you add filler by punching through the skin over the puddle with the filler rod. Some people pick it up pretty quickly, while others struggle for quite some time trying on their own. Maybe find someone local that can demonstrate for you, in person.
Long arc, short arc, heliarc and in-the-dark!