Pronunciation of 'wh-" words

Hi, I look up the 'wh-' words ( words like 'what', 'when', 'whale'...etc.) in different dictionaries. Take 'what' for example, the pronunciation of it is marked as /what/ in one dictionary; while /wat/ is marked in another. I know both of them can be heard. My questions is: is the difference 'dialect-sensitive'? or related to personal choices? Could it be that one is unstressed and the other is stressed?? Do all of the 'wh-' words come with the two ways of pronunciation??? Or this happens only to some words??? :-?
Thank you a lot.

The /hw/ pronunciation for words containing "wh" is all but dead in England and, according to the Longman Pronunciation Dictionary, it survives perhaps only in self-conscious speech, when people try to be especially careful. In the US, I've noticed it's slightly more common, but in casual speech the distinction between /hw/ and /w/ is almost never heard. It seems to be predominant though, in Scotland and Ireland.