sounds like it may be coming from the fire wall. when this happens my brake petal pulsates and it will stop but, a lot slower than normal. Could it be a caliper binding up somehow? also, the ABS light will come on every time this happens and a lot of times it comes on anyway
Sometimes when I hit the brakes, hear a groaning type noise from the front on 1999 GMC Sonoma
Have the ABS system scanned to find out which wheel speed sensor is dropping out! (I am pretty sure!) Also the wires/connectors give a bit of trouble, most of the time on the right (pass. side) front! The "groaning" noise you hear is the ABS system releasing pressure to the affected wheel, will sound like it is coming from firewall as that is where the controller is. - (It's not the brake pads or calipers!!!)
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Sounds like you need brake pads and rotors turned.