If you have your fan set to automatic - clean your filters and make sure there's no dust inside the case. Dust can clog stuff up, increase heat inside the case and so it will set itself to spin faster to compensate. Maybe also consider what room/where you have it. Is it in direct sunlight? Are you in a particularly hot country? Is it a small room that can get too hot too quick?
If you do reduce the speed by the voltage, please stagger it. Reduce it by a small amount, use it for an hour and check temps and repeat until you have a happy medium. You don't want your computer overheating and maybe causing damage.