Yup. Actually fairly common. I found a few other occurances of this exact problem when I was researching mine. My 4JH-2E(1997) started leaking a very little bit like this a couple years ago. The combo exhaust
manifod/heat exchanger housing corroded a pinhole from the fresh water(coolant) side to the exhaust
outlet side. I could barely detect the smell of AF in my exhaust. The leak got worse and I had to replace the manifold($1,000). Here is a photo
of the leak. The new part has a substantially thicker cross section so Yanmar
knew about this vulnerability and modified the design.