if u meanu can hear the motor whirring, vents blowing with the fan speed dial off whilst stationary, then it could be faulty switch or faulty resistor card..
if u mean the vents blowing air, fan speed dial off and not recirculating air whilst travelling at speed, that's cos air pressure above the cabin inlet vent at speed is high thus pushes air in naturally. turn on re-circ and it should block out ththe airstream thus stop the flow..