I just found the chain is non-standard one. It's wider than 9 speed bike chain but the plates are much shorter and the pins are narrower too.
There is no quick link and it's fairly tight so have no idea how much it has stretched but the teeth on the chainring and cog looks OK, no "sharktooth" wear. Nevertheless, I figured it wouldn't hurt to clean and degrease it and re-lube with regular chain oil. And that's what I did. Noise went down a bit after the clean up and re-lube. I managed to seal the chainguard almost completely so dust will be greatly reduced. Hope it lasts for couple of years more.