This subject was just covered in great detail yesterday in mechanics forum. The big/big method suggests the shortest possible chain. There is nothing wrong with that unless you want to change the cassette to one with a larger cog at some point. The little/little method will suggest the chain length that is the longest possible and it will work with any cassette within the RD's wrap capacity. If you have a setup with the largest possible wrap, than both methods should suggest the same chain length.

http://www.bikeforums.net/showthread.php?t=590356

I always use the little/little method with a RD that has the proper wrap capacity. Then I can use any cassette I want and not have to worry about changing the chain length.

It's quite likely that your chain was previously set the the longest useable length and now it's set to the shortest. Either will work, but the shorter one will certainly not be quieter.

Noise from the RD, cassette and chain can come from all sorts of problems including RD hanger misalignment that requires a special tool to check. It could also be something as simple as improper shift cable tension, a worn chain or using a dry chain lube.