Buzzing noise coming from abs unit on start up, when he noise stops the abs light comes on ?

Depending on what car you have. It's as though you have an abs ring/sensor fault, the noise you are hearing is the abs pump working this is because the gap between the abs sensor and the abs ring is to close, which make's the abs control module think that the wheel has locked up. causing it to open the valves in the abs hydro unit to try take away the pressure from the side with the fault.

Your ebcm internal relay is likely failed, possibly has a c0265 code in ebcm. Pull the abs maxi-fuse underhood fuseblock, abs and red brake light will come on, but noise should stop. There are several module rebuilders online (module masters comes to mind)

howdie sounds like their could be abs module week or you could have a sensor wire broke on one of the wheels, or the master cly could have a leak does the pedal come bk up right away?ck this out if not get a quote from your local garage its not good to mess around with brakes

the abs valve assembly is under the drivers seat mounted on the frame if a gear went bad it will keep trying to move the valves it has a electrical plug on it and you will see all the brake lines go in and out of it

This sounds like the abs system is coming on, this may be due to a bad wheel sensor, the best way to see if the noise is the abs is to remove abs fuse and drive, the abs light will come on but drive to see if noise is gone. by removing the fuse the system will shut down. the way it makes noise when close to a stop means the abs could be the noise, a bad sensor at the wheel would cause this. it looses the signal for a sec or two and the compuetr thinks the wheel is skidding and the rest are moving, so the noise you hear is the abs unit release the wheel. if the abs is the problem then the next step is to have a garage scan the abs module for codes. the codes will tell you where the problem is and which wheel. good day.

the shreiking is probably your belt, which might be loose, and the lights dimming could be the same thing, the belt loose

you may have to plug car up to diagnostics

Per your description, I would suspect you have a faulty wheel speed sensor. Take you vehicle to your local Auto Zone or Advanced Auto and have the codes pulled for free. Write them down and let me know what they are and we can go from there.
Good luck.
Regards,

Your ABS systems use a combination of electronic and hydraulic systems to modulate the brakes individually to prevent them from locking up. ANTILOCK systems consist of wheel speed sensors, a hydraulic control unit (HCU), and an electronic control unit (ECU). Because of how complex the system is your computer does a self-test. If the check is successful, the ANTILOCK and BRAKE lights will go out within a few seconds. If one of the component of the ABS system fails to pass one or both lights will remain lit. One or both warning lights may also come on while driving, also indicating a problem. If just the ANTILOCK warning light comes on, some ABS component has failed, but the car still has hydraulic brakes. The grinding noise could the the ABS pump or your brake pads could to worn to the metal backing and grinding against the brake rotors. I recommend a inspection soon for your safety.

check under vehicle on frame rail drivers side, should see abs assembly, disconnect red wire buzzing will stop, will have to replace assembly, will not have abs braking, will have base brakes, if red brake light comes on do not drive vehicle, have towed to repair shop