Can hear cheeping noise have no idea why!?

ok the strangest thing is happening, in the hall outside my room i can hear a really loud noise like cheep or a squeak, although its not exactly the same noise i thought somehow a bird must have got in, anyway i go out into the corridor (which is very small) and cannot see any bird, i cant even see any insects, i closed the window thinking that maybe the sound was coming in through the tiny gap (we're talking maybe a centimetre here). i can still hear it though. i just cannot work out what it is, because it is very loud but i honestly cannot see anything that could be the cause, and the window was open to such a tiny amount that i dont know what could have got in. the noise is like EEP 30 second pause EEP 30 second pause and so on. i dont even know how to explain the squeak, closest thing i can think of is if you open an unoiled door really quickly, you know, high pitched and sharp. so basically i just want to know if anyone has any ideas on what it is and how i can get it to shut up so i can get some sleep!