While apply the brakes you feel and hear a pulsation through the brake pedal.

Agree with both answers,, also a professional scanner will show which wheel speed sensor is failing while test driving....got to have it scanned!!! Code readers are not going to help.

Can be multiple things. Most common would be a failed or dirty ABS sensor. However can range up to and include ABS harness corrossion (resistance), corrosion at connector, and failing ABS module. In most cases ABS diagnosis is required for a proper repair.

Most likely cause is a wheel speed sensor if failing. Need to check stored fault code with compatable scan tool first.