Civilitas:
Thank you very much for your insight. I agree that it could be air leaking in at the pump shaft. There is no kind of clutch on the pump so when the engine starts the pump starts rotating immediately, but it is bypassing the fluid back to the reservoir at a low pressure. It is not until the control switch is moved to on does pressure build in the system. Yesterday the whine did not begin until approximately 5 minutes after the switch was turned on.
I have a stethoscope and have listened to all the components, the noise is at the pump and in the outlet hose leaving the pump.
The reservoir is above the level of the pump and the fluid is gravity fed to the pump ( as all systems should be). Anything impeding the gravity fed fluid to the pump will starve the pump and it will cavitate, causing a whine.
So I believe either air is being introduced into the system, or the pump is starving for fluid
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