Check the start capacitor

[Ken Layton]

Anytime the compressor just sits and hums, it's always a good idea to unplug the power cord. Now get out the multimeter, discharge the start capacitor by shorting across it's terminals with a screwdriver, now check the capacitor for shorted or open condition. Many times this capacitor will have failed due to old age. Replacements can be found in your local W.W. Grainger branch office.