Hvac blend door actuators.
Symptoms: bubbling, murmuring, buzzing, sound, noise behind dash on startup, after key off, often mistaken for improper coolant system bleed, air pocket in heater core.
The difference being the noise happens on key on, and after key off.
There’s two actuators on the rhs of the passenger footwell, on the hvac airbox which contains the heater core, ac condenser and the blend doors that actuate when the mode button is cycled.
Two potential issues:
The tab on the blend door motor has popped out of the track/cam that moves the blend door: reseat it and test the mode button.
Or the motors position sensing is worn out.
Carefully remove the motor, replace it with a working item new or s/h.
Or open the motors gear case. Slightly bend the actuator position sensing fingers inside to fresh track.
Add some quality, plastic friendly grease, probably silicone grease or food grade grease.
Reseal and reinstall the blend door motor and test using the mode button.
Iirc I tested it before I replaced all of the trim, just to be sure.
Iirc this can be a dash out procedure on LHD cars, but always consult the FSM regardless.
Link from the forum first hit:
https://www.google.com.au/search?biw=49 ... 1vrDUmz7z4
Here’s a pic from when I did mine.
viewtopic.php?f=10&t=20939&p=356707&hilit=fixed#p356707
Here’s the FSM workshop manual link on the forum for procedures:
FAQ: Subaru FSM Service Manual Parts Procedures Diagnostics • club.liberty.asn.au
viewtopic.php?f=6&t=34980
Tools: trim removal tools, a good 1/4” metric ratchet set with hex bits, sockets and extensions, phillips screw drivers in short and long.