I have a 2005 Subaru Outback and the brakes make a scary pulsing-grinding sound when I am slowing to a stop while driving or backing down a hill. It only lasts for the split second that it takes me to release and reapply the brakes, then it is gone. It just seems like the kind of thing the mechanic would shrug and go “Mueh?” at.
The symptom you describe is absolutely normal for ABS brakes engaging. ABS(anti lock brakes) makes brakes pulsate when activated.
If this is happening on a slippery surface like ice/snow or gravel then it is perfectly normal. If this is on pavement it sounds like overactive ABS.
That all being said I own the sister car (lowered Outback) 2005 jLegacy wagon and it does not have overactive ABS whatsoever. The only time I notice something like that is on a slippery surface especially when I had the factory Bridgestone tires.
I like Andrew’s suggestion, but you might want to consider warped rotors.
I agree that this sounds (no pun intended) like the ABS system is activating.
Whether the system is being “overactive” or whether it is acting normally depends on the road surface. If the roadway is wet or has any loose gravel on it, then the system is probably acting normally.
If this phenomenon is taking place on a dry, paved, non-slippery surface, then the system is being overactive, thus indicating a problem that should be attended to because it is lengthening the OP’s stopping distances.