When I first start my car and sometimes at random intervals driving it, my ABS control unit make a clicking noise, almost like a fan hitting something (but obviously not). I am 99% positive that it is coming from the ABS control unit as I can feel it vibrating when its making the noise. I can also feel the clicking/virbations in the brake pedal when I can hear the noise. Like I said, its very occasional and only lasts for 30 seconds at a time (or so). The brakes work well and none of the dummy lights are on in the instrument cluster. Any ideas what might be causing such a noise?

I searched on quite a few different terms and couldn't find much related to this problem.