The pressure switch activates the solenoid valve on the unloading valve.
The solenoid valve blows off but stops venting.
The compressor continues to build pressure and eventually causes the safety valve to open.
If I remember correctly, on this machine there is only 1 solenoid valve, which is installed directly onto the inlet valve assembly.
Please see the picture below. This the most common type of unloading/inlet/blow-off valve setup.
Sometimes the solenoid valve is separate from the inlet valve, and connected by flexible hoses, but especially on newer machines, they are bolted directly on the inlet valve assembly.
(and older machines often have separate blow-off and loading/unloading solenoids, this is often so with Ingersoll Rand machines).
When the compressor switches to ‘unload’, this solenoid closes (it’s of the normally closed type), which makes a valve in the inlet valve assembly open, for blow-off.
At the same time, the inlet valve itself doesn’t get any pressure any more and should close.
The venting sound that you hear coming from the solenoid, is the control air that is trapped above the main inlet valve piston (directly under the word ‘inlet valve’) so this is still working, and it looks like all the air is vented, there’s no leaks, but the inlet valve is stuck in open position.
This can happen sometimes. Mostly because of dirt and/or worn rings on the main piston.You will need to remove the inlet valve, disassemble it and clean all the parts (best get an overhaul kit and put new rings and gaskets in).
Be careful when opening up inlet valve, they contain a big powerful (compressed) spring.
Hope this helps!