bubbling (water flowing sound behind the dash) in the heater matrix prob nothing to do with headgasket. Mines does it, did it before the rebuild, after the rebuild and has done for 2 years i've had the car, my old wrx did it as well.
If it's not using fluid, overheating etc etc then i suspect your worrying about nothing 🙂 By all means sniff test it but i suspect its fine.