Only time I've heard similar noise was when a door was left open where it shouldn't have as security type alarm..
That noise has been going on intermittently for years - sometimes at 2 am. It's definitely from the Hornsey rail workshops.
Train reversing @ Hornsey Depot?
Could it be the wind warning alarm on tower crane beside the station?
Unlikely since it can be heard in all kinds of weather. It definitely comes from within the train servicing depot.
It sounds like a "yodel" alarm used on level crossings. There are a couple of crossings in the "new" Hornsey depot layout I believe.
Strange that they would make noise within a depot as it's a low train speed and all users would generally be au fait with the concept of a train... although the entire road complex seems over-engineered within - traffic lights protect a single track stretch on the bridge over Turnpike Lane...
Alternatively it might also be a movement alarm for equipment within the depot building - overhead cranes etc.
It's the noise of the trains being serviced and it's gone worse in the past 2 years...