We'll assume that the hiss was not there before. Could be an exhaust leak or a leak or restriction in the air induction system. Would help to know if you experience the hiss from the driver's seat or outside of the car.
Subaru Outback was created in 1995.
It may be the heat shields on the exhaust system vibrating.
The Subaru Outback is sold in the US.
The 2006 Subaru Outback has 16 valves.
The 2013 Subaru Outback has 16 valves.
The 2002 Subaru Outback has 16 valves.
The 2001 Subaru Outback has 16 valves.
The 2011 Subaru Outback has 16 valves.
The 2005 Subaru Outback has 16 valves.
The 2007 Subaru Outback has 16 valves.
The 2004 Subaru Outback has 16 valves.
The 2014 Subaru Outback has 16 valves.