nyuh
Usually spoken three times repeatedly after viewing something ultimately cute or pleasing.  It's the sound you make when ordinary words can't properly describe your appreciation of the viewed object's cuteness or awesomeness.

Often preceded by a drawn-out "Awww."

Frequently confirmed by a singular repetition from the other person.  
Person 1:  Hey - look how cute that puppy is!
Person 2:     Awwww...   nyuh nyuh nyuh!!!
Person 1:  Nyuh.