The temperature sensor is the brass plug that sits between the two spark plugs.
It sends a signal to the warning buzzer if it detects too much heat during an overheating head condition.
But older sensors do wear old or become less accurate. Try testing without the sensor wire plugged into the sensor. If it stops, then you have a bad sensor. Do not simply unplug the sensor and forget about it - get a new sensor since it is designed to let you know if you really do have an overheating engine before it seizes.