Re: Buzzing High F
At the lower end, you can even rattle key guards and such. It's also possible to rattle a spring that is loose or in poor contact. The action might seem light or even ok, but in the rested position, the spring isn't firmly engaged with the keyrod.
Just a couple more ideas.
Sometimes, the only thing to do is just disassemble it part way or fully, cleaning things out while you're in there, and putting it back together again. Gotta be gentle, though, or you can do more harm than good if you're not used to that sort of thing.