the car sometimes make splash sound when I start in the morning.
Check the coolant level
If it’s low, what you’re hearing is coolant/air passing thru the heater core.
Tester
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If not the coolant being low, I’d check to see if the carpet is getting wet / damp below the dashboard in case your A/C isn’t draining waste water to the exterior. Also, check the cowl area outside just below the windshield for accumulating water, and of course if you have a sunroof check the area around that roof opening.
Oh, wait! Here’s a good one… My son has an Outback just like yours and the seal around the “third” brake light was leaking and it turns out to be fairly normal for these cars to have a faulty gasket there that disintegrates. I’d check back there under the mat and make sure the rear hatch doesn’t slosh because of water inside.
CSA 
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Is this when you start the engine or when you start moving? The other answers below are all good. In addition, if only when you start moving, check for water accumulating anywhere in the doors and lower body panels as drains can become clogged by dirt or rust. Sunroof gutter overflow from plugged drains also can show up odd places - up high, in front, in rear.
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