Anytime you have critters in your house it is because they have found a way in and you need to find that entry point and, while they are out and about, seal it so they cannot get in. A 3 year old house should not have any such opening and in your case the builder should come back and fix the problem, but you do not want any critter wandering around inside you house because they can cause serious damage.

squirrel girl, I cannot add anything to kimmsr's post but you can try running some temporary lighting up there. For many years I lived in a house that was outfitted with fluorescent lighting in the attic. Every 3 months we'd run these lights full time for about a week to chase and keep out bats. Once you have had to deal with a section of the ceiling collapsing under the weight of bat dung you will remember to run those lights!

Yes: Exclusion. And a loud radio, first, will get rid of them.

Let's talk more about the potential for problems! If you own this home, you should jump on this problem right away. Look at ronalawn's example, yikes.

If they are rodents of some kind, you could be talking about a population explosion if allowed to carry on without control. Damage to the actual structure is likely, as well as to your insulation, wiring, plastic pipes, etc.

Animals bring their own little parasites with them, too! Mites, fleas, ticks, etc. NO THANKS!

Hire someone who will get up there in the daylight, without the help of a flashlight. Sometimes the gleam of sunlight will show clearly where the gaps are.

I believe that exclusion is the best way to go, rather than resorting to poisons. But you must do something or the situation will only get worse.

(Don't even think of mothballs, by the way. They are more hazardous to you than most things around the home.)