Your ABS Accumulator is failing or has failed.

On power up the ABS ECU does a self test from time to time and that is the noise your hearing, you can feel the brake pedal vibrate as well.

When the accumulator has failed the ABS pump runs longer to try and build up the pressure however it cannot and the ECU detects this and registers a fault code.

Fault code 1-8 (Accumulator Gas Leak) in ABS memory confirms this.