why there is no ghuna on noon of dunia.

Question
why there is no ghuna on noon of dunia.
why  there is no nazal sound on dunia.
Whats the rule because there is ya is not a throat letter and also ya is also not in laam and ra'?  Thanks

Answer
The idghaam of the noon saakinah and tanween into a letter of the is done between two words, not within a word.  There is no idhgaam here, instead the noon is pronounced clearly.
The rule for the words for the noon saakinah in the four words is called  , which means absolute clearness. 
Please see:
http://www.abouttajweed.com/the-noon-saakinah-and-tanween/merging-of-the-noon-saakinah-and-tanween-p.html

You are welcome.