That's a nuisance but hopefully you have caught it in time. I guess as it is fine at slower speeds and I assume with a good tell tale at idle you are discounting the impeller? To be honest I'm a simple things first kinda guy so would inspect/change that first.
However in truth it's probably more likely to be something above that such as stat or waterways. On the 9.8 the stat is behind the head so you need to pull the head off to check which also gives you access to that area of waterways. Also you can pull off the exhaust plate (items 12/13 in linked diagram) which gives you further waterways to check/prod.
So just head gasket, two exhaust plate gaskets and a stat to buy.
This job is straightforward as far as it goes with minimal tools required. As long as you remove all the bolts with care to avoid snapping then you should have no problems.
Parts diagram here gives a good idea how it all fits together...
https://en.impex-jp.com/catalogs/out...mbly-4556.html
I'd like to get a maintenance manual for mine, ideally hard copy, but so far no luck.
