Subaru's heater is very difficult to get all the air out of, so CHECK your coolant level straight away, if you are losing coolant, the heater is the first part you notice it in, I have known this noise on Legacy's that have taken weeks to bleed the air out of the system, the noise is exactly as you describe, its water flowing from one side of the heater matrix to the other.
Check your coolant level now in case you cook your engine!!!!!!!