Weird noises behind fireplace...rats or mice?

well i have a covered up fireplace and i keep hearing weird sounds like i suppose it sounds like something sleeping, cant really describe the sound like soft beebing, like a baby animal would make. does anyone know what mice sound like at that age? would this be mice, rats or birds? it sounds like its just behind the hard carboard cover, but its securely in place, i cant open it. I never hear scratching noises, only that really weird sound, its been like that for a month now....doesnt really sound like lots of animals are making it, more like one. when i tap it it sounds like its woken up and making a noise if you understand what i mean. i just tapped it there and heard a metal clink kinda creepy lol