As an undergraduate (a long time ago!) I did a project on whistlers, which are natural VLF noises produced as a result of VLF emissions being dispersed along magnetic field lines. I nearly did a PhD studying these. There are lots of different natural sounds at VLF. Whistlers are just one type. Tweeks are far more common. I recall going out to quiet parts of Wales to make recordings, although tweeks could even be heard in the centre of Liverpool after the manmade noises subsided late at night.
See https://soundcloud.com/nasa/whistler-waves2 . Eerie noises and not manmade.