What is the primary indication that an air brake system compressor unloader valve is functioning properly?

1. A loud continuous whistling noise inside the cabin
2. A distinct 'psssht' air purge sound from the air dryer purge valve when the system reaches cutout pressure (approx. 7.5–8.0 bar), followed by compressor unloading
3. Engine immediately stalls
4. All dashboard lights flicker off