The question really, is when is it satisfactory and when is it not? The board performs fine - if you were deaf you would notice no difference I assume. But still, it is a defect of sorts (if it exists, I'm being hypothetical here) and as a consumer surely you'd want a non-defective product.
Is a high pitched whistling noise defective enough to be counted as 'faulty' or 'grade b' goods? I'm not the one to judge, but I would say if it was my computer I'd want it swapped too as I do like a bit of silence. High pitched noises are infinitely more annoying than low, so I can see how it gets on your nerves.
Just both have to play the diplomacy game so you both end up happy and give Asus an earbashing too. This kind of thing puts people off a name quite quickly, and I think it would be unfair to lay the blame squarely on Scan when they have in this thread been quite productive in trying to rectify the problem.