Well, I don't have Rostock, but I'm planning on building one in the next few months. Assuming you're using LM8UU's, you could give this a try: [
www.thingiverse.com]. I have cheap LM8UU's on my Prusa, and they are definitely the greatest source of noise from my printer, so I want to try out those igus bushings. The other thing that helps (at least with Cartesian bots) is a reduced acceleration setting. Also, I think there's something on Nophead's blog about shorting a certain resistor on the Pololus to make them run quieter. Hopefully one of these helps you out!