I am having a problem with our Ducane AC Unit outside. When I turn on the thermostat control, the "system on" doesn't come on. Then the outise unit, would make a humming sound for a second or two then stops, but the fan works. There is air coming out of the vents but its not cold. What could be happening? I called an AC guy and he said it might be the compressor then another guys said it maybe the capacitor?

