What is making this sound?

Dennis,
My 383 made similar sounds before it overflowed. I suspect your antifreeze was past the boiling point. Remember, water boils at 212°, but the radiator cap compresses that so it actually can get above that temp before the spring under the cap allows it to overflow.
Sometimes my big block wouldnt overflow until I shut it down, when the water stopped circulating and the fan stops cooling it would build up enough heat (pressure making sounds) to eventually overcome the spring in the radiator cap. Then I would later see a small puddle beneath the drip tube.
Make sense?
After it cools, check your level in the radiator. If you can see antifreeze, your good.
I always squeeze the upper radiator hose first, if you feel pressure in that hose, dont open the cap, its the same pressure in the upper radiator tank.
Joe