That is your temperature sensor telling you your engine is running hot.
Do this, start it up using your flushing hose and when the beeper goes off, feel the water. If it's really hot, where you can hold your hand in the stream for more than three seconds, your overheating. If the beeper is going off and you feel the water and it's not hot. Then your temperature sensor is bad. Replace it.
Let me know if it's actually running hot and is not a faulty sensor. If it's hot, let me know and I'll help you narrow down what your problem might be.