ABS question

I just purchased an used van and just noticed that when the van's brakes are used for the first time that the anti-lock brakes engage and make a noise but then from that point on the brakes work fine. Any clues??

I have often noticed a very slight sound of my ABS when backing out of my driveway....and apply the brakes.
I may be wrong....but I have always thought that it was a self test.
I figure that if it was a problem.....it would do it at other times....and not just the first application of the brakes.
So, I would not worry about it unless it starts doing it beyond the first application of brakes.