Eureka! Finally got the case open on WD MyBook II
Instructions on WD site @ http://www.wdc.com/en/library/usb/2779-701026.pdf (on page 19, chapter 7) are not exactly correct. You have to work some tabs on the case before you can (gently) pry the case apart. You need to gently pry around the tabs.
Looks like there is a small Sunon Maglev (KDE1205PFVX) 50x50x10mm fan (rated 12v, 2W, 6100rpm, 35dba, 15.2cfm). No wonder this sounds like a jet engine!
Also, the opening for the fan to push air into the drive cage is small 40mm x 15mm (approximate), so most of the fan is pushing air against the sheet metal.
Wondering is there is a quieter fan to replace this little monster. Might also work better by placing at the bottom to push air straight up thru the middle of the case directly thru the fans?