Once the nada is heard, slowly pierce through it until a softer sound appears. Focus on this new sound. Then go past the new sound and another one shall emerge. The nada is inside the mind only and the sound is to be heard in the mind only. We cannot create these sounds, they are already there. We have to attune our minds for the nada to manifest.
These subtle electrical vibrations are commonly felt especially during meditation and pranayama. Learning the Khechari Mudra is actually easy. The more important element here is the spiritual preparation in which the guidance of the teacher will be of huge benefit.
When the Kundalini is awakened properly, it could be directed to enter the Sushumna nadi where the chakras of the subtle body exist. She irrigates her pathway as she makes her way up, loosening the knots of ignorance known as the granthis, which in turn illuminates the soul.
The tongue holds the key to experiencing God. Energetically, the tongue has a direct connection to both the origins and endpoints of the energetic channels, from the hips to the inner brain, from the Muladhara chakra to the Sahasrara chakra.