not bad. depends how good the pump is
some are and some are not i know that the v2 400 skimmer is silent because when i went to the lfs they had a tank setup with one on it and it was not very loud. there was a bit of noise. but if you had the tv on or if it is in your bedroom you could still get to sleep and you could still have the tv on with out a load of noise coming from your tank