Normally generators are not very loud.. it depends on the generator capacity. Example: 2500 Watt Max Generator running only 800 - 1000 watts will not be too loud at all. If you were running maybe 1800 - 2000 watts, it will rev A LOT HIGHER to achive that power.
If you are talking about running just a main pump, tunze, and skimmer then.. it should not be that bad at all... IMO