If you've just set up your aquarium, you're going to have to get used to the sound. You should only really be able to hear it when it's totally quiet, like when you're going to sleep. If it splashes excessively, raise the water line until it's silenced. If it makes a loud rattling, groaning, or any other noise, unplug it immediately and recheck the assembly. The impeller is usually not shoved into place quite right.
If the aquarium has been up and running for quite a while and gets louder, odds are the impeller is dirty. Take it out and rinse it well. Sometimes, trash will get in between the impeller and its axle. Pop off one of the axle caps and wipe off the axle and run a lot of water through the hole in the impeller. The cap can be shoved back on. This can make an enormous difference in the noise level.
Airpumps can be quietened by placing them on a small piece of foam rubber, or they can be suspended inside an old panty hose so that the pump doesn't touch anything.