Click Personal Profile after login to opt out. I know of two situations where flowmeters indicate a cyclic change in flow of large magnitude in a period of seconds. This strongly suggests that the cyclic change in flow is real. I once designed a flow loop for demonstrating check valve operation using a centrifugal pump, check valve, a tee with a ball valve and vertical leg full of air, and a ball valve on the discharge leg. While I have never seen any references to this in any books, my guess is that you have gas in your system, or possibly pulsation dampers that are undercharged, and it is acting like a spring-mass oscillator with the pump acting as the excitation force.
Heat exchangers can also have a large amount of flexure if there are enough tubes.
You might also check for pressure relief devices that are set too low. Look at the pump performance curve and the discharge pressure, if discharge pressure is less the the discharge pressure at the BEP, this could be you problem. The name by which you'll be known in the forums. Click Here to find out why. Selling and recruiting forbidden.