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Welcome to Automotive Forums Pheezie! You have asked a difficult question to answer for your first post. Short answer is yes it could be a freeze plug. The best advice I can give you is this: With the engine cool, you will need to feel around the the hoses underneath on the right side to start...where the crossover pipe from waterpump connects to heater hose, (all connect points) top radiator hose connects to thermostat, look carefully at housing base. Sometimes you just can't see but need to feel for wet areas to pinpoint source of leak. Leaks can migrate around engines in strange patterns so are very hard to find. There is a weep hole in back side of waterpump try to feel around there as well. If that is source the waterpump bearing seal is bad and will need to be replaced. You mention you flushed the system and I am assuming you did this yourself and replaced with dexcool and did the proper bleed valve procedure when refilling. Just try to eliminate as many areas that are simple to fix first. One question I have is mileage on your 3400?