believing that you checked the obvious - which is torque converter drain plug not loosened out.

The noise you hear is the system running low on oil - specially if you have a splashing leak.

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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)