The ABS cycles the valves sometimes when you start the car, to keep them free and working...you hear a crunching/chattering that seems to come from down by the base of the steering column...is that the sound you hear, only when the brakes used?
If you don't get the "Low Trac" blue light on the dash when it happens, I'd think it was an uncalled-for valve cycle...if the ABS was actually active, you'd feel it in the pedal...
I don't know the remedy, but it does seem to be a common problem...
I think before I gave up on it, I'd take it out on some loose gravel or something and try to skid it around and activate the ABS...to see if it works on demand, and perhaps if activated it might reset whatever is making it malfunction...
I believe mine has held up so long without problems because I force it to activate regularly...whenever there's snow, ice, or wet I'll give it a few stomps and make it work...keeps it from getting atrophied I think...don't allow the fluid to stagnate in the system, etc...