May peace be with you.
The 'un' that you refer to is due to the 'tanween' (i.e. in this case, the two dhammas on the last letter (double vowel) or known as 'dhammatain')).
The 'tanween' renders the noun 'indefinite'. An indefinite noun is one that isn't specific (i.e. can refer to any). For e.g. kitabUN (a book) can refer to ‘any book’ as opposed to ‘The Book’ (al-kitabu) which is definite and refers to a specific book.
Unless the word appears mid-sentence, the 'un' sound that you refer to is not vocally articulated.
I hope that helps, God willing.
Joseph.