PFU, the manufacturer of HHKBs, has said it’s “normal” and it goes away with time. Some users have said their units did in fact stop whistling, whereas others have continued to experience the problem. When I asked Brian at EliteKeyboards, he said the problem can disappear over time… but it can also come back again.
Some have said it’s the “Topre fart” effect at work, caused by air escaping from underneath the Topre domes, whereas others disagree and believe it’s due to friction. At time same time, lubing the sliders doesn’t seem to help… so…
Basically, no one knows what’s going on. As I see it, there is speculation and assertion, but no convincing evidence that isolates the one variable causing the problem. The upshot, of course, is that there’s no known fix. If you buy a Type-S right now, you’re gambling on whether you’ll receive a nice, quiet board, or you’ll feel like you’re living next to miniature train tracks.
Really, it’s on PFU, not the community, to sort it out. Whether or not PFU is acknowledging it as a defect, let me be clear: it’s very annoying — annoying enough that I’d recommend staying away from the Type-S until it’s been reengineered to fix the problem. The whistling sound is definitely more pronounced in real life than in the typing video. Not what you’d expect from a factory-silenced board that costs more than $350.