I'm tempted to say never but that's probably because I have no idea what you mean, lol. "eng"? what sound is that supposed to make in french? the "un" sound is exactly the same as the "ain" and "in" sounds. UN is pronounced the same way as mAIN, pAIN, vIN, etc... listen to the word "vin" here: http://fr.forvo.com/word/vin/ "un" is pronounced the same way. (without the v of course). no exceptions that I know of.

People in south of France pronounce "un" a little bit differently from people in the north. They will pronounce the letter "n" at the end of "un".

well, I'm a native speaker and I pronounce them the exact same way. haha. what difference do you see? I just said them out loud and there's none, lol.

I am not a native speaker of french, but to me it seems that 'vin' and 'un' are a little bit different, but they are quite similar in that they have that same nasal sound.