Afaik drives with less platters are usually less noisy.
I don't know if they sell in retail yet but Toshiba offers a 500GB single platter 7mm drive for a while now. If you put 2-3mm isolation on top and 2-3mm below it would probably be as quite as it gets.
On SPCR they used to use Bitumen build HDD isolation. Not sure what is the best material. Obviously one that is cheap, easy to get, transmits enough heat and silences as good as possible.
I am quite sure that putting some kind of sound isolation on a thinner drive helps more than anything else. The biggest problem is that the hdd is very close to your ear and the only thing between is a very thin Aluminium layer that doesn't really do anything for sound isolation.
Termal shouldn't be too much of a problem as an hdd radiates most heat through the metal casing on the sides and there won't be space to put something there anyway. Yet a 2-3 mm layer of a dense enough material on top most definitely help some.
I do have a Toshiba 320GB 5400rpm drive in my 2010 MBP. It is by no means loud and has very silent seek noises but it is a worlds difference between it running and being off (when only the SSD is in use). The HDD is still in the HDD bay and the SSD in the optical as there the HDD would probably be noiser with all the supension gone and everything.
Or this one
http://www.storagereview.com/hitachi_travelstar_z5k320_review
also available in 500GB
http://www.hitachigst.com/internal-drives/mobile/travelstar/travelstar-z5k500