i use on fX5 on my 75 gallon, and even when the ac isnt running and the tv isnt on, youd never notice the sound. my father visited the other day and remarked on how quiet it was
thats dabbling into the world of amplitude and noise frequency analysis.. perhaps if the impeller was rotating at the same frequency of resonation of, say, the stand the tank was on, or the hood, (which is HIGHLY unlikely), then perhaps it would seem louder. 2 filters are most likely going to sound more muddled, and not necessarily louder.