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+1 on "listen".

Up and down should have the same sound....
Changes clues that there may be a problem (or not).

As for tension lay the razor closed on the strop and
tighten to the point that...

The zipping sound is the spine running over texture on the
canvas side of the strop (I am guessing Illinois Razor Co. Strop??).

My guess is that your X stroke rotates the razor to a slight...