Knocking noises on startup are disconcerting, but are not always something horrible. Especially if the noise goes away in a short while. Really big problem noises like bad main or rod bearings make noises that do not go away.
Drain the Dex and refill with "mixes with any type or color" coolant according to the instructions in Techinfo http://www.bonnevilleclub.com/forum/...=article&k=100
If you really really want to use old green phosphated coolant, (I can't imagine why you would) you better be absolutely sure you have removed all of the Dex from the cooling system first. Green and Dex will cause big sludge. This includes the heater core, engine block, and radiator.