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You're right: the singing is explained here: http://www.classicshaving.com/articles/article/590351/9362.htm

It also tells you at the bottom that non-singing razors are discarded.

That's the high pitched noise when stropping. Some razors do it anyway some only when they are stropped with the spine lifted from the leather.
TI razors are famous for it. I have once heard that TI...