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noise from central air unit

My central air does cool the house. However, I started noticing recently that the outside unit has been making very loud buzzing noise, like metal banging or strong vibration. While the system is running, I switched off the outside unit's power (the switch is next to the outside central air unit). At this time, the fan turned off, and I presume everything was turned off, yet, the noise persisted every few minutes. Is it possible the thermostat inside the house is sending a signal to turn the compressor on, and the control unit - whatever it is - is making the noise? Can it be easily fixed? I would like to know the cause before calling for service. If it is serious and expensive to fix, I might replace this 20-year old unit. Thanks!

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