I was yesterday in a bus in the HK neighbourhood called 上環/soeng6 waan4. A guy nearby was talking in a strange, almost alien-sounding, Chinese dialect. At some point he mentioned (what I suppose is answering a question) "tseng wan" -- with the "w" being weaker than a normal [w]. I didn't catch the tones but "zeng"/上 sounded much higher than in Canto. I'd guess it's his dialect's version of 上環. It's not Hakka, I think. (上環 in HK's Hakka is song van). According to this site, 上環 is something like sèng vãn in Toisanese. Close but not quite right.
The guy also spoke Standard Cantonese, which is why I suspect it's a Southern dialect.