Motors causing HC-SR04 ultra sonic sensors to receive incorrect duration.

I was building a Arduino toy car which looks like this

and below is the motor shield I am using

The ultrasonic sensors work just fine when the motor is turned off. However, they start to receive duration around 0.05 when the motors starts turning. The problem stil remained when I tried isolate the arduino UNO from the motors (i.e. lifting the UNO with my hands).

The manual of the motor shield says that the pins that are free are all the analog pins and pin 2, 9, 10, 13.

I really have no idea how is this happening. Please help!

Motors are electrically noisy. The best way to have the motors not effect the Uno is to have a separate power supply (battery pack) for the motors.

for all motors add 100μF capacitor across power & ground leads