I have a 2003 Subaru Legacy that with heat set to max, the temp is just a little warm.  The engine temp is in the middle and stays that way after engine warms up.  I get cold air in the summer with the AC on but in the winter with heat it's not very warm even at max.  When I purchased the car, it had a new radiator and thermostat put in along with head gaskets replaced so that part is all working well.  I do hear some gurgling from the heater core infrequently.  The coolant level is good.  I felt the hoses going to the heater core and the upper one was warm, not hot, and the other felt air temp.  Any suggestions?

Flush heater core