Update: Well, it looks like I was wrong, it wasn't the feet after all. It just happens to be most of, and only, the keys closest to the feet--when I took off the bottom of the case the noise was still there when the keys were pressed. I suppose I just have the dreaded HHKB switch spring ping on a few of my switches...
When keys closer to the adjustable feet are pressed (1, w) the case makes a noticeable pinging sound. Further keys mean fainter sound. The noise comes from some sort of spring in the feet. I initially thought it was the keys or springs but I have ruled that out.
This happens whether the feet are extended are not. It also happens when the case is tapped on. And it happens regardless of surface.
Anyone else have this problem? Any fixes? I have already realigned the case as carefully as possible. I actually don’t remember having this problem before I did, but I might have. Perhaps someone can try their hhkb to see if they have a similar problem.