Why can I hear the sound of the flow when the outlet pressure is higher than the inlet pressure?

a. Dirt or damage on sealing surfaces

To do:

1. Clean the parts

b. The valve disc is catching due to wear, dirt, rust etc.

To do:

1. Replace the check valve

c. The coil spring is dislodged or broken

To do:

1. Replace with a new coil spring

d. The spring holder dislodged or broken

To do:

1. Replace the check valve