Ghunnah
A nasal sound held for two counts (about a half-second). Comes from the nose with the mouth closed.
When you see a shaddah (gemination mark) on a meem (م) or a noon (ن).
Close your lips for meem or rest your tongue tip on the upper gum for noon, then hold the nasal hum for two counts before moving on.
The doubled noon in 'inna' is held with a nasal hum for two counts.