Lagge, scorpioub,
What you’ve heard are called hypnagogic hallucinations. HH can be visual, auditive or tactile.
About hypnagogic sounds, we maintain a collection there:
The list of hypnagogic sounds
The explanation depends on the sound heard. For instance, voices are due to the disconnection of the prefrontal cortex areas, so that thoughts are heard as if they were pronounced by another people. Weird sounds like buzzing, popping, etc. are due to something like the synchronization between the 2 brain lobes. They are linked to brainwaves changes (delta waves?) and as far as I remember, some of them (buzzing) have been reproduced while subjecting brain to a magnetic field.