@medtro
I am experiencing the same noise. My motor and pump works as you can hear it pressurizing up the accumulator. I believe that the high pressure switch, which is within the black box of the abs unit, is faulty. When it's cold out, the switch sticks? causing the motor to run the pump abnormally long and eventually over-pressurizing the accumulator. This is where the squealing/squawking noise is coming from. I'm not sure if the brake fluid is back flowing through the pump or its being diverted through a safety valve but that's not the root cause so I'm not digging deeper. I haven't cracked opened the black box, but I've seen photos from another member and I don't believe it's replaceable. Having said that, I'll be replacing my abs booster assembly in the next few days.
Btw, once warmed up, the high pressure switch works fine. I have been depressurizing the accumulator by pumping it 20+ times with the ignition off whenever I'm done driving for the day. This allows the motor/pump to run a bit longer upon startup. I'm guessing the fluid warms up a bit while being pumped/pressurized and allows the high pressure switch to "warm-up". But the high pressure switch has been sticking(getting worst) on colder days so I'm just going to change out the complete assembly.
Take a look at the 2nd link for FSM instructions to check the pressure switches. My high pressure switch had high resistance when pressurized.
Abs issue some work done but stuck?
is this sound the accuator or the pump on the brake system?