Bird deterrents, high pitch squel that apparently humans can't hear. Does my head in.
I remember the first time I heard one of those, I was out cleaning with my dad, I said to him: "what the
modded is that noise?" "Can't hear anything" he said. "What do you mean you can't hear anything!?! It's really
modded loud!"
Something like ten years later, I've not heard any for a while, I figure they either didn't work (they never seemed to, the houses that had them still got bird
modded on their windows) and were abandoned or the squeal drove the neighbours bonkers and they were shut off. Anyway I'm out cleaning with a new start, he says to me "what the
modded is that noise?"