Heater core was replaced, hoses that connect heater core were fabricated after install. I think it sounds like water sloushing and I detect a faint odor of anti-freeze. Would hoses make a difference? Or could replacement core be defective.
You may still have some coolant on the new core. Make sure the hoses aren't kinked. The noise you hear leads me to believe there is air trapped in the system or the coolant level isn't full. Make sure to bleed the system fully.