Out of the mouth of our Guru. If you have room in your robot, get some Dynamat off of eBay and give it a try.

@feroze if you can't do dynamat then I would look into high torque analog servos. In areas where I simply didn't have extra room I found analog were quiet in comparison. Here's a video of two that I had to modify for continous rotation for gearboxes and also a video comparing analog vs digital.

Here's a video comparing accuracy but you can use this as a reference of what difference analog vs digital makes. https://www.youtube.com/watch?v=ILKEAgL-a9w&feature=youtube_gdata_player

Here are two analog servos close up and you can hear about how much noise they make ( not much) https://www.youtube.com/watch?v=uTi_U3ogIy4&feature=youtube_gdata_player

I can understand your concern , my hitec 5995 ltg servos make a loud buzzing that would drive me nuts even under the lightest load so I found they would not be what I want for my robots shoulders. I went with a analog servo and a gearbox to multiply the torque. You can usually search YouTube by model to see how a servo performs and sounds. Digital will always buzz at least a little and is part of how they work but a analog in some cases can make much less noise. Its all preference.