Gray green substance under oil cap - Cummins ISX15

[SupVit]

Also there is another change of engine work.
Engine knocks when I push throttle pedal.
The heavier the moment on the engine, the louder the knock. Quieter at 1100, increases significantly after 1500-1700 rpm (when heavy truck climbs the hill). Almost every day when I pull a heavy load in the mountains, at the end of my shift I hear the engine works louder. Something deteriorates faster under heavy load. Knock is almost absent at idle.
Absent during engine braking going downhill. So the problem somehow intersects with the operation of the fuel system
The knock frequency is 2 times less than the engine speed (I recorded the sound of the engine, slowed it down 4 times and counted the knock frequency). The knock goes, it seems to me, just above the compressor. Maybe the head gasket near 1, 2 cylinders ... And now I think maybe, because of the gasket, there is a mixing of fluids. And knocking is combustion high pressure. It's time to take the head off and look for the problem.
The knock began to appear in the summer of 2019. I already went to the Cummins dealer to test the fuel system, they did not find anything. Went to Rush Center - same result. Went to experienced mechanic -same. Few thousand dollars wasted but nobody knows. It looks like it's time to just open the engine. Probably it's the same problem...
There is about half an inch of foam in the coolant tank. I thought before it's okay, because liquid flows into the tank and foams. But now I think foam is here because of gases going into cooling system through the gasket.
Somehow everything goes in logical chain too easy :-))