A 10-20 gallon with a Sunsun 603B is dead quiet as long as the output is under the water. I have one on my 20 long and it's the perfect thing for quiet efficient filtering of small tanks. The smaller 602B is usually only like a dollar cheaper than the 603B but they have the same motor and flow rate so stick with the 603B unless you absolutely can't fit the slightly larger one.
There are a few HOB filters that are near silent but IMO even those are a little more noticeable than the little Sunsun canisters.
There are a few HOB filters that are near silent but IMO even those are a little more noticeable than the little Sunsun canisters.