Open the reservoir cap and see the fluid with engine running, if its like a whirlpool in there, the reservoir filter is broken - the turbulence of the fluid is causing this noise.

replacing the reservoir tank will fix this.

mostly the failing part on this system is the lathe cut O ring that seals the tank to the pump. If that leaks the result is usually noise as the fluid is aerated as its being pulled into the pump.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)