Same problem here, I have 2 WD HDD, Gold 2TB and Black 2TB, they both do this sound on power down, and I have 4 other WD HDDs that are ok, no strange sounds from them, I am trying to get an answer from a technician, but they keep sending me to troubleshooting page (sounds, test and so on…) that gives no results…
Here is my thread: https://community.wd.com/t/when-hdd-power-off-it-makes-moaning-noise/218183
