Too much noise from a Koralia 240 probably means there is something wrong with it. Mine only made noise if I took the outside magnet off, then the suction cup would still hold it up, but the interior magnet would vibrate against the glass. How this is your glass? I am now using a Tunze 6040 which is slightly bigger and a lot more expensive, but has variable flow from 53 gph to 1150 gph and has stronger exterior magnets available if you are mounting on thicker glass. The Tunze is very quiet. There is a single speed version, the 6020, that is the same form factor (so stronger magnet still works, and it is much cheaper than the 6040) but is probably too much flow for you, I think it is over 600 gph.
-Justin