How to find out whats living in my attic?

Strange noise keep coming from the attic. My husband has set traps and poison in case it might be a mouse but nothing has taken the bait. Bit the noise seems to loud for it to be a mouse anyway. We have checked the attic over and over again with no signs of anything, which makes me think that the animal is living in between the walls of the ceiling and the attic. How do I find out what it is and how to get it out? We have checked the exterior of the house and cant figure out how it is entering. Any experts?