This can't be good. Coolant was obviously leaking back by the transmission. I was relieved to discover that it was coming from topside, but angry at the thought that this could be the manifold gasket recently replaced. It was not. It was a $5 piece of plastic. I learned that the connection at the manifold for one of the heater hoses, had a quick disconnect. It turns out that piece of plastic had broken, and under high RPM like I was using while towing a full trailer, coolant came spewing out. Why ? Why do you need or want a quick disconnect there ? The coolant has an overflow tank and a drain. The radiator has a pressure relief valve. So why ? Anyway, the plastic was replaced and coolant contained.