Many people have noted that most of pump noise can be quietened down by doing two things - dumpening vibrations with something like shoggy sandwitch or petras gel stuff (not too aplicable for pumptops integrated into bay res) and to prevent extra noise from air bubbles hitting impeller by completely bleeding from air loop (sometimes it's not done completely and sometimes big flow might reintroduce bubbles via whirl in reservoir, so some extra steps/countermeasures might be needed, eg. turning rad upside down to get air bubble out, or putting some antiwhirl inserts in reservoir, and/or topping water level in reservoir). Some noise might stay anyway, and of course 3.25 is noisiest one DDC as it's most powerful (maybe apart from DDC2).