when the HDD in the computer shuts down, there is about 3 secs of coil whine. I have an Asus z170-a motherboard, Samsung SM951 m.2 SSD (OS drive) and WD Black 2 TB HDD.
That is unpleasant. 🙁
You mean that your HDD is making some weird noises right? Are you sure that sound comes from the HDD? If you open the side panel, you may have a clearer view.
If this is the case, then my suggestion would be to back up the data stored on the drive until you sort this out.
After that, you can attach the drive to another SATA port with different SATA and power cables. If the issue persists, you can run both short and extended DLG tests on the drive: http://products.wdc.com/support/kb.ashx?id=5M3o1C