I have a standard Lessco 200gal skid with Hypro D-30 pump 5.5 honda 300' hanney elec reel. The pump is less than year old just replaced relief valve assembly (bail lever type). My problem is at about 100psi I get bad knocking sound. Oil in site glass is good and clear not milky,air pressure is 80 psi 20% less than desired operating pressure (would like to get higher pressures) relief line is unobstructed. Should I be worried about knocking sound? it doesn't seem healthy for pump or possibly me. Any ideas or thoughts.