yeah, I've checked exactly like that and the news are not very good :(
the front sprocket is worn quite a lot. And even I've seen some piling of metal from the sprocket, sprayed around the place (like small sparkling "dust"). The pins are "shark fin" shape. So that is quite gone. And I don't understand why as the wheel was aligned right and the chain was lubed.
Also the chain is tensing so it is worn uneven. Basically in a full rotation of the chain, there are 2 main "positions":
the chain is either very tight/tensed in a spot or de-tensed for the rest of the time.
That is what was making the noise: when I've adjusted the chain last time, it must have been on the "lose" position... which resulted in a very tight chain when it revolved into "tensed" position. So at each revolution it was "hurshing" and making that awful sound.
Now I've readjusted the chain so that even in tight position it still has the 1.5-2 cm slack. This of course resulted in a much larger slack in the "de-tensed" position but I think this is a better situation than having it very very tight in the tensed position.
The bottom of line: I need to buy new chain and sprockets set after only 18000 kilometers. And I've really taken care of the chain and lubed it and adjusted it. I think next time I might as well try to leave it be and not clean/lube it.
Now the question is: how safe it is to continue using this chain until we get out of "middle of nowhere" and into a bigger town where we will have a chance to change the chain?
And that is a big question, as I would really hope that the chain is strong enough not to brake and allow us to get out of here.