When does realisation of velar nasal /ŋ/ as alveolar nasal [n] happen along with tensing of the preceding vowel (/ɪ/ to [i])?
Solution 1:
As @JohnLawler points out, it would take an extensive sociolinguistic study to arrive at something definitive. Based on various bits of research provided in the comments, this accent appears often in speakers from California who perform a "velar pinch." I'm marking this answered because I think until a deeper study is done, this is what we have:
- http://dialectblog.com/2011/11/10/the-western-us-and-velars/
- http://phonetic-blog.blogspot.com/2009/11/i.html
- http://www.stanford.edu/~eckert/vowels.html