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For starters, turn on the car with the hood open and see if there's any obvious source of the sound in the engine compartment. If you don't hear anything from there, turn off the fan and see if the hiss goes away to see if it's coming from air duct
I have the same thing on my GS300.
I managed to locate it around the fuel tank - it seems to be coming directly from it.
I just assumed it was the pump being loud, and stopped there.
Can anyone else check? It's coming from just below the rear door.
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