I have a l35 right now. When you hook two of them up in series, they make a ton of noise. IF you only have one hooked up, it is near silent. I have never used a l40 before, and infact did not know they existed untill I found it on ebay. I am definatly looking for a quality pump. I am going to try to watercool the entire comp (HD, PSU, CPU, GPU, and anthything else in my way). To do this I am going to need a strong pump because I am making an external box that is going to house two 77 bonniville HCs along with the pump and maybe one of the blowers I keep reading about. Thanks for the ideas on the pumps so far. The only thing I have against using pumps in series, is that everytime I do that, one pump starts to rattle and get really lound. I have tried with ehiem and hydor. Not sure if the D4 is different. I also just kinda want to worry about hooking up one pump instead of 4 (saves time, effort, and most importantly money).