Weird "heartbeat" like noise with WD Green 3TB ( WD30EZRX )

Can you repeat the wdidle3 command and confirm it’s been disabled?

I need to source a similar type of drive to see if it’s due to APM. Alternatively, can you try to disable APM (yes I’m aware that 3.5" does not come with APM) with hdparm and see if the parks continue?

WD30EZRX and aggressive head parking