for me i dont think they make noise at all if you put them on 600!!! but on 1200 they might make a little humming sound very low!!!! i have 2 600 and 2 1200 and i dont hear any noise!!
I can't comment about the 600 or 900's, but I have the 1200. You won't hear them if you're playing music or watching TV, but if its dead silent in the room and you're trying to sleep, you definitely hear a hum. I own 2 different versions and both do the same.
I use it for mixing salt now and it works great
i have to put my ear on the tank to hear them!!!! maybe see if the impeller cover is on tightly!!! all i have to say is im very happy with them!!! aqnd i have no died spots!!!!
ho ps i am not useing the magnet on them i have them on the sucktion cup brakets and that might make a diffrence also!!!!