Bats!
This is a bit of a saga, so I’ll do a quick intro before I get to the guts of it. I purchased a house in December 2006, and after a month or so living there I noticed a scratching noise in the walls. I pulled out the air-con, and looked into the wall cavity, and there was a roosting bat. So I did a bit of research into them, and decided they were getting in around the air-con, so I left the air-con out one night, and late in the evening sealed up the hole, and put it back in. Of course the next night there was still scratching, but not only there, in the ceiling as well. ...