Noisy AC Delco PS Pumps

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2011 GMC Yukon: I've replaced my PS pump twice in the past month with new AC Delco, OEM units and the second pump is still noisy. I've flushed the system completely, used Valvoline PS fluid, replaced the pressure and return lines, used new seals on the lines at the reservoir and steering rack, and bled the system at least 5 times. The pump still whines especially when turning. There is no sign of air in the reservoir (bubbles or foaming) nor any sign of fluid leaking. Are these made in China pumps that unreliable or am I missing something?  

 

There could be air pulling into the system at one of the hose connections without a visible fluid leak but I would think that would result in air bubbles in the fluid. 

 

Any thoughts are appreciated. I dread the idea of pulling this a third time and dealing with the inconvenient Rockauto return process. 

[alvocado]

To close the loop, the pump was defective. Pump #3 is dead quiet. It's unfortunate that 2 of 3 new AC Delco pumps were duds but I guess that's what we get with inexpensive made in China parts.

 

For anyone considering buying from Rockauto, it's not the best experience if you encounter more than one warranty issue. You have to pay for shipping for both replacement parts and to return the defective parts. Worse, there is no option to request a second warranty replacement for a part that is still within the warranty period and Rockauto offers no email or phone customer support. I spent an additional $25 shipping parts and waiting for days which was no cheaper than buying locally. A better option for auto parts is Amazon which is free Prime shipping, they cover shipping both ways on warranty parts and they don't charge you for a replacement as long as you return the defective part within a week.