I have MCP655-B's (non-variable) in all my systems, except for a MCP655 (variable), and a MCP35X (variable). I've also tried the MCP355 (non-variable) and an X2O 750 pump/reservoir combo (non-variable).
MCP655 and MCP655-B's are near silent, but it can take a while until all the bubbles/air are out of the loop.
MCP35X is the next quietest, though slightly audible, but can be adjusted for lower RPM's and thus, as quiet as the MCP655;s.
The MCP355 and X2O 750 are the loudest, the MCP355 with a high whirring sound, and the X2O 750 with a slight whirring sound.
The type of reservoir and location of the pump/reservoir combo can also help determine noise/vibration levels. Overall, the MCP655's are the best, can't even hear them. I use the 5.25" Dual Bay Reservoirs (for Laing D5).
Edited by El_Capitan, 24 October 2012 - 05:53 AM.