whirr

To whirr is to make the low steady sound of something spinning. A fan, a hard drive, the wings of a bird taking off.

The sound is continuous and mechanical, which is what separates it from a buzz or a hum, and it always involves rotation. British English doubles the r and American English usually writes whir. As a noun the whirr of a projector is a standard piece of description. The word is imitative, built directly out of the noise it names, like buzz, hiss and clatter.