The only way you'll decrease the noise of the WD Greens is to encase them in a good mechanical housing like the Smart Drive or Scythe Queit Drive (neither of which are cheap). But those will re-introduce the vibration-induced noise... unless you can somehow suspend the enclosures. Not easy, due to the large size.
You could also trying blocking the front vent directly in front of the drives where the noise probably comes out to you the most, but that will also tend to make them -- and other components -- run hotter.
I see what you mean by not cheap:http://www.quietpc.com/products/harddri ... ve-classic
The scythe seems to be non-existent on UK shores (where I am from)
I had good results with the custom enclosures, which are basically just metal boxes with a couple of gel-packs around the HDD to dampen any vibrations and transmit heat from the HDD to the enclosure. There's a thread on here somewhere with details.
I may look into this. Id rather buy than diy, but if the advantages>disadvantages, it will likely sway me.