Factory fitted compressor had an intermittent valve. Replaced with brand new Nissens OEM equivalent. New compressor resolved a number of issues including cold air at all times and less noisy engine and viscous fan now operates at all times and variable speed.

The concern I've got now however, is this compressor makes a loud howl, like a supercharger, when it's engaged, which increases in loudness with increase in engine RPM and goes away instantly when AC button is turned off and comes back immediately when turned on as if the AC button is an on/off switch for the howl. Also with this howl is a resonance/vibration which I can feel in the floor and gas pedal.

The original never did this so I reckon perhaps a fault of some sort?