If you are sure you don't have any leaks, I suspect something near the inlet/filter is flapping in the breeze. Press on different parts and see if the noise changes.
It can help to hold one end of a paper towel roll (or other piece of tubing) to your ear and move the other end around the engine compartment until you find the source of the noise. Watch out for fans, belts, and other moving parts - it's easy to get distracted when troubleshooting.
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