My central AC unit isn't working. The fan seems to always spin at a low speed even when the air isn't on. When it tries to kick on, it hums a bit and the fan speeds up to its normal fast speed but the compressor fails to kick on. It sounds like it wants to but fails. I already replaced the contactor with no effect. Could it be the capacitor, or does any of the symptoms ring a bell to anyone?

I would say a new capacitor and a new motor.