A: If the motor connections are properly set, then you need to check the actual voltage supplied at the motor. It is best to do this with the motor, and any other lights or equipment that share that circuit, running. Using a voltmeter, check the voltage across Line 1 and Line 2 leads. It should be within 5% of the nominal 115 or 230V. It the voltage is low, correct the problem before trying to operate the pump.
With the pump running, slowly close down your valve on the discharge side of the pump. If the noise goes away, then you were hearing cavitation. Cavitation is caused when the pump isn’t getting water re-supplied quickly enough. To remedy this, you need to
1. Reduce the output by partially closing a valve on the discharge, until you reach a point where the cavitation stops.
2. Increase the diameter of pipe on the suction of the pump.
3. Shorten the length of the pipe on the suction of the pump.
4. Reduce the numbers of elbows on the suction of the pump.
5. Lower the elevation of the pump.
Any one or combination of the above changes will help eliminate cavitation.