I have a 1989 John deere 444e getting small bubbles in overflow tank. No white smoke No water in oil Starts and runs great no blow by No bubbles on cold start up No overheating No exhaust gases in rad test Seems to bubble more on load, then less when at idle. If i fill the rad an inch below the fill neck it will push coolant to overflow. If the coolant is 2 inches below no coolant pushes out. Just bubbles into overflow. Head gasket, cracked head or head lift on compression? Any one this machine with the same problem?
If you send out an oil sample it should tell if you have a combustion leak into the cooling system. There is also kits like this: https://www.blocktester.com/product/block-tester-bt-500-combustion-leak-test-kit/ I had an engine doing something similar, it turned out to be the water pump.