Sounds like you are due for a flush and install new anti freeze. But, be fore you go that route, I would take it out for a good drive fore a couple hours and see what develops from that. Usually stuff like that is a result of oil or combustion getting into the cooling system.
A pressure test of the cooling system would be a good idea to see if there are any leaks.
Could also be sludge from years of work and just needs to be cleaned and replaced, but, ya never know.
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2019 Unity LTV CB, pushed by a 2013 Honda CRV, BlueOx Baseplate, Aventa Bar & Patriot Brake
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