I cannot find any difference between the íng sound in líng and míng. ing sometimes sounds like like ieng. The vowel e between i and ng may be closeto the oo sound in good.
That is, there is a hidden vowel in ing. The i is a consonant rather than a vowel.
I'm not sure if it is the hidden y you mean.
Some speakers may prefer pronouncing ing as-is. i is the only vowel in their ing.
Honestly, I can't hear any difference in the 'ing' part between ling and ming, and I don't think there is a 'y' sound before or after 'i'. That said, maybe some speakers would raise their tongues a little bit after producing the 'l' sound, resulting in an approximant sound similar to 'y' but it doesn't sound standard to me.
Chinese "i" should always sould like "yi" or [i:].
If you hear someone says "ming" without the "hidden y" you referred to, either the guy speaks very fast, very casual, or he's not a native Chinese speaker.
I think the difference of "íng" in the two words that the OP heard is probably due to the speakers: the two words are by two different speakers with slight different ways of saying the sound "ing".
(All learners should note that there are variations of "ing" by Chinese speakers from different regions.)