I always hear un noise at the end of the word but why is not it waladun instead of walad
Yes that looks like a mistake to me too. Unless we're missing something?
The "-un" at the end of words is used to indicate that the words are in the nominative/subjective case. "Nunation" is used in MSA but not in Arabic dialects. Sometimes Duolingo uses nunation, other times it doesn't.
I'd like to know how Duo lingo had tapped into my email database. Lately when i try type a word, it pulls up random email addresses rather than the word I'm trying to type. Bit suspicious
I think it's your keyboard or keyboard settings in your mobile, laptop or PC that has tapped into your database. Check the setting. JazaakALLAH