Or there's a loose connection. The BC21 is wired point-to-point and some of the components are supported by the wiring. I had a similar problem with a BC 21 which deteriorated into more noise. Cured with one dab of soldering iron. Open it up and look for something loose, probably connected to volume control.
have a knowledgable tech spray the volume pot for you with an oil-free cleaning solvent (& then work the control end-to-end several times before the spray dries - tech should know to do this). If the symptoms continue then move on.
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