Compressor not kicking in?

Read your description twice, because the useful clue is the one that looks like part of the story: the compressor sound went away and you could still hear the fan and piston. That rules a lot out. A dead motor would be silence. A thrown belt (if yours is belt drive) would leave the motor turning and the piston stopped. Your pump is still turning and still not making pressure, so the problem is in the air path, not the motor.

The first thing I would look at is the check valve between the pump and the tank, and the unloader on the pressure switch. When that check valve sticks open, every stroke the pump makes gets pushed straight back out through the unloader. The machine gets quieter, sounds like it is just spinning, and builds nothing. You described that almost word for word.

Four free checks before you buy anything. Unplug it first.

1. Drain valve shut, and the tank not sitting full of water. It sounds stupid. It is also a real cause.

2. Run it 30 seconds, shut it off, stand still and listen. A hiss at the pump head, the pressure switch, or a fitting tells you where the air is going.

3. Pull the hose and cap or plug the tank outlet. If it builds pressure now, the leak is downstream in the hose, coupler, or regulator.

4. Tank empty and unit unplugged, unscrew the brass check valve at the tank inlet. It should pass air one way only. If you can blow through it both ways, or something rattles inside, that is your part. Usually $10 to $20, ten minutes to swap. Thread sealant on the tank side only, and do not overdo it or shreds end up in the tank.

If the check valve is good and it still builds nothing, go to the pump head. Four bolts, and under it are the head gasket and the reed valves. A torn gasket or a chipped reed is obvious to the eye. Kits run $10 to $25. With the head off, look at the cylinder wall and the piston for oil blow-by. Scoring or a wiped ring means a pump kit or a pump, and on a 1.5 hp unit that is the point where you price a replacement instead.

What I would not do: buy a new compressor before the $12 part. Open the pressure switch or the motor housing for wiring unless you are comfortable around mains, and never with it plugged in. Run it with the head off to see if it pumps.

My read, from your description only and not a hands-on inspection: leaning hard toward the check valve or the unloader, and a fix rather than a replacement. If you tell me where the hiss is when it shuts off, pump head or pressure switch, I can narrow it to one part.