If a drive is mounted, you can eject it. That will also spin the drive down. Ejecting is the same as unmounting a drive.
Probably not a good plan if the drive is part of a set, such as a RAID set, etc - but a simple formatted drive that is not your boot drive can be ejected.
Just right-click on the drive and choose Eject from the menu.
You can also unmount individual volumes in Disk Utility - where you can mount the drive again if you want.
I would try gently moving each drive sled, pushing at different angles. You may hear the buzzing change, or even disappear completely when you move the "offender" slightly.
If you are OK with that - you can also pull a drive sled completely out - assuming that you are certain that you are pulling an ejected drive. You may not need to do that, if the noise quits when you eject a drive.
Double-check that there is no disk in the optical drive.
(Bet it turns out to be a fan, not a drive )