My Schwinn S-600 sometimes vibrates when cruising at 12 mph. If I back off the throttle the rapid pulsing stops. Replacing the motor, roller clutch, chain, controller, and tire did not help. Coasting is smooth and wheel bearings are good. The red throttle light does not illuminate during these episodes, only when the batteries are getting low. The intermittent vibration does not affect functionality nor top speed of 22 mph, just very annoying. Your thoughts please.
Battery Pack: 36V B&B 12V 10AH (provides 40 minutes and 7-10 miles)
Motor: 24V 750W XYD-6B (increases speed) with 15T free wheel sprocket
Controller: 36V 30A XK-022D with 5-wire throttle