Braking full stop while descending a ramp, rattling sound - possible brake issue?

I recently acquired a pre-owned 2012 GLS V AT, 37k odo, while going down a steep ramp , I stepped on the brake and had a full stop, gear still in D position, pretty sure I heard a rattling sound and felt a strong vibration. Has anyone experienced a scenario like this, what could be the possible problem - if any? Sana wala naman po. TIA