Are you sure, there is no signal in background radiation? Someone should check.
Pure radio static is the sound of the universe! Very frequency dependent - the higher up in frequency (or shorter the wavelength) the more of it is cosmic in nature - point a microwave radio dish towards/away from the sun and hear the difference, or even down towards the ground to hear the sky/earth noise. [1]
I personally find a clear spot between military submarine stations in the low frequency 10-60 KHz range quite relaxing to listen to as it's a mix of random noise and thunder static.
Go VLF or ELF for earth sounds;