What could be causing the filler tube of a toilet to be leaking/bubbling?
I've run into this same problem several times, and tried the same things you're trying - adjust the ball, bend the arm a bit to put more pressure on the valve, etc.

Ultimately though I've always ended up replacing the fill valve. The pressure from the float ball and arm isn't enough to close the valve completely, a seal is worn out, or something else is similarly out of whack.

ETA: In my experience, the fill valves that don't have a ball but instead have a sort of integrated float seem to be less prone to this problem (like the one pictured below)