I also agree that the whisper line of products are totaly silent.
You'll probably need an air stone or something to move the surface though..... my whisper 10i did a great job cleaning but it didn't have enough flow to break the surface so i kept getting a film on top of the water.
I've got a penguin biowheel on there now (the small one) and it's silent too as long as the water is level with the output.
Ohhh if you do get a whisper internal, don't buy replacement filters...
Remove the old filter floss/carbon from the filter frame and cut a piece of filter material to the same size and stuff it back in, it worked 100x better then the original, soaked up alot of crap.
Chris