It sounds like steel under stress just before it pops. However this sound is probably overlaid on the primary audio track because it sounds like it is not part of the normal background noise and nothing else is stopping. If it was really loud enough to be heard by the camera then it would be deathly quiet with no birds, people/children playing or whatever. Lots of things would stop doing their normal thing IF this was real.
this... always look to nature for your first instinct. if birds are flying AWAY, then something isnt right, if theyre sitting there doing theyre thing all is good.
