I believe that Nevada is correct about this noise merely being the ABS self-test.
On some models, this normal electronic routine is so quiet as to be virtually unnoticeable.
On other models, it can be very intrusive and can involve a slight trembling of the brake pedal, coupled with knocking sounds that may sound like they are coming from the rear, or from underneath the hood. It really does vary quite a bit from model to model, and can even be slightly different with two different samples of the same model.
So, in other words, this self-test can range from quiet and unapparent, to quite noisy.
And…if someone has never previously driven a car with ABS, it can be particularly unsettling, especially if that person has not read the description of this phenomenon that is found in the Owner’s Manual. Just out of curiosity…is this the OP’s first car with ABS?