knocking and surging

My 2014 Prius 3 with 245K miles has been bucking and surging for some time. No check engine light, power and gas mileage are fine. The bucking only happens during steady speed. Not accelerating or decelerating. At around 30 to 35 miles per hour I hear a steady knock that corresponds with the bucking and continues as long as the bucking. A mechanic told me that was the switch that switches between the electric and gas modes. Could this be causing the surging and if not how much of a problem is this?