Quieter, faster continuous rotation servos
Any standard PMDC motor would work for this, brushed or brushless.  

Continuous rotation servos if I recall correctly are basically just a hack.  The typical servo has a radial pot, motor, gearing, and a little controller to get the position right.  Their goal is to provide repeatable, precise control, not be efficient or very fast.  The reason they make so much noise is because you have a tiny, tiny motor spinning a massive step-down transmission.

Using "just a motor" and gearing it down yourself (or with an integrated gear head on the motor) is the best solution.