Sometimes the bearings can get worn, and this causes friction, which in turn can cause the whining noise you have. Some people get around this by sqirting, a tiny amount, (better to squirt onto finger and let it seep into fan) of oil between the joints of the fan (make sure only on plastic bits and only on fan), I personally would just get a new fan.
I don't have this fan, but I would think that the reason the noise varies is because it is temp controlled and so is only working at high speeds when doing intensive work like gaming, video, and graphics, etc.
This may all be wrong in your case, but is a poss answer. Hope this helps.