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...
I finally got to the bottom of this - response from Thameslink:
I am very sorry to say that as the noise is part of the vital safety system of the depot, it won't be possible for the noise to be disabled or lowered. There are Rail Safety and Standards Board (RSSB) specification on issues like this and unfortunately we have next to zero flexibility. The noise is a warning sound linked to our wheel lathe machine which is located not far from your property. The alarm is activated from a panel on one of our train tugs to signify that a train movement is about to take place. As there will be a certain level of noise created by the train movement it is essential that the alarm is of a certain level so it is clearly audible over sounds associated with the train movement.
Unfortunately, most depot work is undertaken during the night. During the morning and evening peak we will have almost 100% of our trains in active service or in standby outside stations. Between the morning and evening peak the excess rolling stock is normally parked in railway sidings across the network. It will only be towards the end of service that trains will be returned to depot and engineers will have a chance to get to work. You can find a depot will be empty with very few staff around during the day then a hive of activity overnight.
Good sleuthing. A reasoned response, even if it's not a reasonable level of noise...
If you're so inclined to view your aural nemesis, you'll see one of the offending vehicles from the Harringay-side steps to the bridge at Hornsey station - as TL said, it drags trains through the wheel lathe building (which isn't electrified for obvious reasons). I saw one doing just this on Saturday with the siren warbling away.