The contactor chatters when the thermostat is OFF, but It makes fine when ON?

The contactor on my Goodman air conditioner (Model CKL24-1K) chatters when the thermostat is OFF, but It makes fine when the thermostat is ON. The contactor and capacitor are new. Turning off the power to the transformer at the breaker box eliminates the chatter (and all electricity to the contactor). The thermostat is a ritetemp. I have not checked the transformer output or the voltage at the contactor (I am overseas currently). Any ideas why the chatter occurs only when the thermostat is OFF? Thanks Dustin