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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.
In a 2004 Subaru Outback, the thermostat is located on the bottom of the engine on the driver's side. Remove the screws holding the fitting that the hose (from the bottom of the radiator leading to the engine block) is attached to, remove the fitting, and the thermostat will be underneath.
On my 2001 Subaru Outback Wagon H6-3.0, it's on the driver side(L/H), in front of the wheel well, hidden behind the fog light...
The Data Link Connector (DLC) is under the bottom part of the Instrument Panel, on the driver's side, by the door...
It's located in rear engine compartment on the driver side. A metal can connected with two rubber hose. HC
The relay is located inside the car, under the steering wheel in the driver's footwell. It is difficult to describe its precise location. However, the Chilton's Subaru Manual has an excellent description and picture in section 4-2.
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You can find the knock sensor on the top side of the engine under the intake manifold. It is easiest to find from the driver's side and is at the back of the engine block.
There isn't a high beam relay but there are headlight relays located in the engine compartment. There are two, one of the driver side and one for the passenger side.
They are in the same place on all 2.5 Subaru engines. Back of the intake manifold, driver's side. This forum will not allow pictures, otherwise I would show you where. Rest assured it is easy to find.
You can open the driver's door and press the "unlock" switch on the armrest. This should unlock all the doors, including the rear hatch. If it doesn't, your solenoid that operates the lock needs to be replaced. For that, see your Subaru dealer.
Assuming you are asking about an automatic transmission Outback, the dipstick for the transmission is accessed from under the hood on the driver's side back close to the firewall. It is a bit low and obscured by hoses that pass above it. Look for a yellow loop handle. You fill the transmission through this dipstick tube.