I've used an Eheim 1260, MCP600, D4, MCP350, MCP355, D5, and now an Aquastream XT. Of all of them, the XT is without question the quietest. It all depends on how sensitive you are to noise/frequency. No offense, but when someone says their D5 is quiet, they are not sensitive to noise. The D5 puts out too high of a pitch for me. The XT isn't cheap, but it performs well and really is very quiet.