[PhantomGremlin]

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.

[TwoNineFive]

Well, like you said, you don't even know how fans work.

[Doxin]

Fans implement variable speed through PWM which is a square wave and therefore rather noisy.