No idea...sent link to thread to cat tech support:
[FONT=&]It  might be the seals or valves leaking internally causing pulsation. Also  is the air is getting in the inlet side it will cause cavitation and  hammering.[/FONT]
 
 
 [FONT=&]Thanks,[/FONT]
 
 [FONT=&]Derrick Rewerts[/FONT]
 [FONT=&]Technical Sales | Cat Pumps [/FONT]