Small correction for the circuit:
Connect 100nF capacitor across DC motor not across 9V battery.
Add the 100nF Capacitor
Place the 100nF ceramic capacitor directly across the motor terminals (in parallel).
One leg of the capacitor connects to the positive terminal of the motor.
The other leg connects to the negative terminal of the motor.
This capacitor helps reduce the electrical noise generated by the motor and smooths out voltage spikes.