Car has a roaring sound when A/C in turned on

You may be having a problem with your blower motor,try to see if changing blower speeds changes sound of the noise. Also if you operate the heater or economy function the compressor should not come on but you still can run the blower motor.The only other thing that comes on with the a/c is the cooling fan,any of these can make the noise you have discribed.If all these are good your looking at a compressor,orface tube and acumulator to repair it.

Its the AC control knob the we use to tell the AC which direction to force the air. Dash, floor, winshield defrost, etc. There are little tubes connected to the back of that knob that passes through air. Just replace the knob and it should fix the noise. :-)