Hmm... things I didn't pay enough attention to when I first started! There are two models of 8TB, 7200rpm (WD82PURZ) and 5640rpm (WD84PURZ). It seems that the 8TB 5640rpm could be the sweet spot for lower power / noise.here is a WD84PURZ part number that i have. its an 8TB 5640 rpm deal.
I'm contemplating swapping my 10TB WD Purple Pro with my 10TB WD RED 5400 RPM, and use the Purple Pro drive as a parity drive for my snapraid volume on the NAS. I can turn off the drive when the parity isn't being generated. A bit disappointed - I should've paid more attention before getting the purple pro. I was focusing on the claim that the Pro offers better reliability.