Had a question from a driver this morning about their brake system and some odd but normal behavior. He has a new house and his commute changed. He noticed the past few mornings that by the time he was halfway down the street his brake pedal would faintly thump and then buzz. He only noticed the problem in the morning.

This is perfectly normal for some ABS and ESC systems. Each time the ignition is turned on and the vehicle is driven at about 16 mph, the system performs a “Self-Check” to confirm components of the system are operating correctly. 16 MPH is typically when passive wheel  speed sensor generate an accurate signal. This “Self-Check” function exercises some of the valves in the modulator body.

This self checks are nothing new. The 1971 Oldsmobile Tornado True-Track rear-wheel anti lock system would make a loud double thump after start up that sounded like a someone was trapped in the trunk.