Oscar's question is right on track. I'd also add...do you have a check valve in the system? In the first diagram Oscar posted, I'd move the check valve to a position right before the accumulator tank. It's possible that your pump pressurizes the lines, then water leaks back through the pump...which then "growls" at low speed to pressurize the lines again. Essentially, this is a leak that you cannot see. The accumulator tank and a check valve should resolve all your issues.
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