If you want it quiet then you will need to lower you fan speed as well as pump speed. My PC is definatley more quiet with fan speed at the lowest as well as the D5 pump on 3.5 speed instead of 5. I use rubber feet on my pump top/res mount and that also helps. My case is also silenced with dampening mats but despite all these measures I can still "hear" my PC when gaming, mainly the corsair fans they have a sound of a diesel engine and the pump has several noises.
In other words use quality fans that are quiet and push good amount of air on low speed as well as rubber feet (if you go with an EK top/res you get included rubber mount). Different top/res has different noises, I had an old EK D5 top/res and it was pretty quiet, this alphacool I have now have some noise that is impossible to get rid of despite being mounted with rubber feet, I think the sound resonates in the tank or something, it gets even worse when the water level is above 2/3 in the tank. Manufacturers definatley need some RND in the noise department because this alphacool is just a joke.