2007 Subaru Outback - Sound of water in pipes

Sounds like water running through pipes in motor after just having heads milled and gaskets replaced

My guess is that you have air in the cooling system. What does the temperature gauge indicate? Have you checked the coolant level since the work was done? If it’s low, top it off. Run the heater on high for a while, then let the engine cool off and check the coolant. The level will probably drop. Top it off and repeat. If none of that gets rid of the sound go back to the shop that did the work.