Kitchen Small Electric Appliances - Microwave Vent Required?

I have a Whirlpool over-the-range unit that is currently set-up to vent out the roof. The problem is the vent pipe produces a massive echo that is heard through the front of the unit. (Sound blanket around the pipe does not help.)

Since the blower motor can be rotated for recirculation (and comes configured as such from the manufacurer), is there any problem operating it this way? Does code reuire outdoor exhaust?

Thanks for any help!!

Welcome to the forums! Reventing is perfectly fine. I am not too sure about California codes, so check with a local authority before going to all that trouble. It is the only place where I reserve advice.