I have been there with a Volvo MD17C.
Check for scale bridging the coolant entry points as the coolant path enters the two cylinder heads (I have 3). A biscuit-like scale builds up across the round fluid entry points and in the corresponding "mirror" image exit point in the manifold.
Drain the coolant from the barrels and maifold, take off the manifiold, and poke the ports clean with a piece of stout wire. I used an old coat hanger and a water hose to flush the debris away.
Total cost will be two manifold gaskets from expensive Volvo, perhaps $50 in total.
With my olde MD 17C, the coolant used to boil and bubble on shut-down.
I fixed it in 1993, and it's been fine since.
The manifold is cast iron and brittle and shockingly expensive from Volvo. In 1997 they wanted £1300.... at today's exchange rate, $2600 for a new one. Don't learn it the way I did.