Long story, short answer...
I had the same thing. Thinking it was water in the ductwork, I drilled a small hole in the bottom of the ductwork to let the water out... WOOSH, the ductwork was the heater core. Anyway.. that being a mess...
Check your fluid level in your radiator.. when the engine is cool open the radiator cap and look for fluid. Start motor and add water or radiator fluid until it is up near the cap.. A little air in that system can give you the sound you have.