Are you sure it was acoustic noise? I don't know how fans actually implement variable speed. But I can think of several methods that would generate lots of ele

Are you sure it was acoustic noise? I don't know how fans actually implement variable speed. But I can think of several methods that would generate lots of electrical noise. Fan manufacturers have an incentive to build as cheaply as possible, so I doubt they take too many steps to mitigate crap they put out on the power lines and/or radiate from their speed controller.