Michael Gray wrote:

It is usually the sides of the case behaving like a drum.
Try damping both sides with your hands, and see what happens.

That makes sense, but holding both sides doesn't diminish the beat. I
can now *feel* it as well as hear it (~ 1 Hz). Any other ideas on
stopping it? How about that rpm reduction I mentioned?

--
Terry Pinnell
Hobbyist, West Sussex, UK