My clipper is making a loud "clunking" sound, is it broken?

No, the unit is not broken. Rather, it is not properly tuned and as such it is not operating at maximum power. To adjust power (tune), use the end of the cleaning brush or a screwdriver to turn power screw in a clockwise direction until you hear a noise. Then, slowly turn the power screw out (counter-clockwise) until the noise stops. This is maximum power.