Buzzing in the brake pedel when comming to a stop, below 8 mph on 2000 Plymouth Breeze

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It seems the anti-lock system is trying to prevent a skid as the car comes to a stop, like it should. However the anti-lock system should disengage below 15 mph or so as I understand it. I found both rear brake wheel cylinders were leaking and replaced them and it seemed to help awhile. This is an intermittent condition, happening a majority of the time. And is very annoying. I cleaned the hubs and pick-ups at all 4 wheels, no difference. This is a nuisance only. Brake operation is in no way affected. The buzzing is felt momentarily in the pedal and the sound like a farting noise for all to hear as we come to a stop. Been like this several years now
(1) Answer
clearance issue with the sensors or a cracked toner ring. have a shop check it for you.
by the way, abs does not de activate below 15 mph. it goes all the way till it stops.