ABS pump runs every 20-30 minutes

The ABS pump on my Honda makes a noisy buzzing sound when it runs. When I first start the car, I hear it buzz, the ABS light goes off within 2 seconds, then the buzzing continues for another 3-4 seconds. While I am driving, I often notice the same buzzing noise periodically. The ABS light never comes on, but it seems like every 20-30 minutes, the ABS pump comes on for 1-2 seconds. Is this foreshadowing an imminent (and expensive to replace) failure of the ABS pump? The ABS brake fluid reservoir did run dry once a few months ago (my ABS light came on), and this has been happening since then, after refilling it.