Pump very noisy

There are several possible causes for a noisy pump.  First, it is necessary to determine if the noise is actually coming from the pump
or the motor. 

Motor Noise

Motor noise is almost always a problem with the front bearings on the motor.  Usually this happens when a leaky pump seal sprays water onto the front of the motor.  When the water gets into the bearings, it will ruin them in a very short period of time.  It is possible to put new bearings on a motor, but most people just replace the motor.

Pump Noise

Some pumps tend to make a lot of noise.  This could be several possible things:

Pump restriction.  If a pump is running under excess restriction, such as a clogged intake, full trap basket or other obstruction, it will be much louder than normal.

Cavitation.  If a pump is starved for water it will tend to cavitate.  This makes a lot of noise because the pump is working against itself.

Over-Performance.  If a pump is moving more water than intended (has much less restriction on the return than is intended), then it will tend to make a lot of noise.  If, for instance, you run a poolsweep pump without the poolsweep unit attached, then it will make a lot of this type of noise. 

Internal Blockage. Sometimes something will get pulled into a pump that will cause it to make a lot of grinding noise.  Also if the impeller on an Aqua-Flo pump is touching the face of the volute, it will make a grinding noise.