"ng" is pronounced normally [ŋ], but for some people it becomes [ng] when followed by a vowel. What's the phonological rule about this? Is it a common feature of all dialects of English?
Basically, the rule is: when the ng comes at the end of a word, you do not pronounce the g (ŋ), and when it is in the middle of a word and followed by a vowel, you do pronounce the g (ŋɡ). The origin of this phonetic g-deletion is called Ng coalescence. However, some dialects treat two words (e.g. wrong of) as one (wrongof) and pronounce it accordingly. This is not normative pronunciation. In the linked article, Wikipedia lists some of the dialects in which it is common:
This is particularly associated with English English accents in an area of northern England and the Midlands, including the cities of Birmingham (see Brummie), Manchester, Liverpool (see Scouse), Sheffield and Stoke-on-Trent. It is also associated with some American English accents in the New York area.