Weird Sound, but from where?

[Flook75]

Hi all,

For the last couple of weeks I've been hearing a weird sound which I assume is coming from the pipes. Some times it's a short burst other times it can last a minute or so

But I can't track it down to a precise location. When I hear the noise I dont have any taps running, shower or washing machine on. I've attached a short video so you can hear what it sounds like.

I do live in a 1st floor flat, so I did think it could be happening when the flat above is running a tap etc. But other then that no idea

Any thoughts on what this could be would be greatly appreciated

Thanks