friend what you describe started 2 years ago for me. the different frequencies when you listen closely, the pulsating humming etc,
what you hear is with your inner ear.
if you search on you tube the sound of the sun you wil find the pulsating sound. The sound of the tibetan singing bowls reproduces that hum.
Heart of the earth waking up, friend. what's more natural than this? :)

You don't have to be afraid. This is a positive happening. I don't know your spiritual story. But maybe you heard of stopping the mind. Those who try to achieve this.
With what you hear this is simple. But of course don't exagerate to the point of reaching the loud sound. Keep the sound simple at the first levels. Play with it, and learn to tune it. Imagine it as a radio, as music.

When you listen to it with the inner ear, you will not hear anymore regular thoughts which you usually hear in the head. You also regular imagination will not work when you are in that state. I'm talking about doing this when you go for a little rest, or before sleep. staying with your eyes closed and playing with this stuff.

What will happen is that without the mind noise there, you can concentrate now on hearing only the voice of the Heart. The voice of the Heart (intuition) is on another frequency band an you can hear it while hearing the sound.
It is a silent voice in the sense that that is not like the noise of the mind. But it is a voice nevertheless.
It's the voice/vision that let's you remember/relive/feel the beautiful moments in your life, the magic moments. the aspiration for a bright future of this earth. things like that friend.

Seacrh for the book Agni Yoga - Heart. It speaks powerful words about the Heart. I found it by 'chance' when the sound started. and it all made sense.