If your air compressor hums instead of the motor firing up, it's telling you something. The message is that it is quite likely a power related issue that is the problem.
Here are some steps for you to try and resolve a humming air compressor motor issue, from the least complex through to the more complex diagnosis and fix.
The first thing to do, then, is to plug the compressor directly into a wall socket, and preferably one that is supplied by a 20 AMP breaker, and has no other electrical applicances pulling power from the same circuit.
If where you need compressed air is far from the compressor, do not move the compressor. Add air hose to extend the reach. The longer hose will diminish the flow of air to the air tool, but using an extension cord will possibly damage the compressor motor. Better under-supply the air to the tool (since that will not hurt the air tool) than to under-supply the electricity to the compressor motor.
Did this resolve the problem with the humming air compressor? If not, please read on.
If you unloader valve does not work, then the added load of air trapped over the piston may be enough to prevent the motor from starting. See the pages on this website about what the unloader valve is and what it does if you are not sure, and then check the unloader valve on your compressor.
A good indication that it is the unloader valve causing the humming air compressor motor is if you completely empty the air tank before trying to start the compressor, and after doing so, the compressor motor starts and doesn't hum. If that's the case with your compressor, you need to get the unloader valve fixed.
Air compressor still humming when it's trying to start?
Unplug the air compressor, open the tank drain and the PRV valve, and drain the tank completely.
Remove the line from the pump head where it reaches the tank, and typically you will find the tank check valve there. It's a one way valve. Blow through it both ways. Air should flow easily in one direction, and not at all in the other. Wipe your lips! :-)
If the check valve isn't working properly, quite often it only needs a good cleaning. Rinse in a clean solvent a few times, let it dry thoroughly, and reassemble.
See the troubleshooting page and follow the link to testing capacitors to find out if yours is causing the problem. If so, you will need to replace the start cap.
If the start capacitor is good, you are probably looking at a pump failure of some kind that's backloading the motor so it cannot start, or the compressor motor itself is failing.
Tearing down the pump means pre-ordering a valve plate and gasket kit. Otherwise, once you have the pump disassembled and you find a problem, you are stuck until you can get parts. Sometimes kits are not available and you may have to cobble up valve plates and make your own gaskets. Information on how to do that is found on this website. Use the search box to find details.
Testing an electric motor means removing it from the compressor and taking it to a shop. In my opinion it's not enough for them just to add power and watch the motor spool up, there needs to be load on the motor shaft to replicate the environment on the compressor, where the shaft is attached to a crank and there is load. That's why testing the motor is the last thing on the list of diagnosing and fixing a humming air compressor. Good luck with yours.