Most likely Electro Magnetic Interference (EMI) from the DC brush motor in the treadmill coupling into the sensor circuitry.

Get a battery operated AM radio and tune it between stations.

Turn the treadmill on and off and see if you detect a change in the noise from the radio.

If the noise is bad enough, it may even be able to be heard when the radio is tuned to a station.

If you can identify wiring for the sensor you can try and wrap it with some aluminum foil.

Even without grounding the foil sometimes you can block enough of the electrical noise to eliminate the problem.