but it my be doing SMART surface tests? see Services->System, smart, Configure
You can try stop smartd, although the drive might be already instructed to do it nevertheless.
You can try also to unplug the network cable for a few minutes, to see if the activity is network deployed, and if the mumbling stops (this only applies if the hdd led blink, otherwise it is internal to the drive). You can also just powerup the drive outside of the box, no sata cable connected, and see if the mumbling also happens.