I think the answer to this is easy but I'm not sure of the part name.
I'm driving along and have the heater on, but it's blowing cold air. Then I hear kind of a "clackety clack", and then the air becomes hot, and I can also see the gauge lowering on the engine temperature.
Seems there is some type of valve which is not opening when it should, but again I am not sure of its name or how to replace it. It sounds like it's under/behind the dashboard in the center area. Definitely NOT the engine thermostat, that's under the engine block and not coming from there.
FYI this is a Subaru Outback 2007 but I vaguely recall hearing this noise and situation in other car(s) before.