I have a 3TB WD Green hard drive, formatted exFAT. The head goes back and forth three or four times quickly, and this event repeats every two seconds. There's absolutely no activity reading or writing. In fact the same happens if the computer is off and the hard drive is on external power with a cradle. I have another HD WD green 4 TB formatted exFAT and it doesn't behave like this.
The hard drive works well, so it doesn't seem to be dying or anything.