Sorry I don't have a fix but this is apparently known:
https://forums.macrumors.com/archive/index.php/t-748874.html
Looks like one guy said his stopped beeping when he selected to not "park" the HD. System Preferences>Hardware>Energy Saver....unchecking "put hard drives to sleep when possible" is the closest thing I can find in OSX SL to what he said. Another said turning off SMS stopped it.
Seems to always be the Seagate drives too.