That is the air inlet for the entire jet air system. There should be a check valve near that side of the tub, inside the motor compartment. If you look into the very hardest-to-get-to corner, which you can only see if you are basically upside down, you will be getting close.
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There may be a section of vinyl tubing (1" OD) which makes a zig zag right nearby. That looks like:
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Undo that check valve, and see what happens. If your noise is gone, put the check valve back into the tube with suction (while the jets are on full blast with all air open). If the noise comes from the valve, replace it. If the noise only comes back when you reconnect the Second side of the check valve, then you have some sort of blockage in the tub from the check valve to the inlet. It is usually wasps, ants, or termites around here. They build a nest and partially block off the tub. If that tube is blocked, put at garden hose nozzle into it and blow it out from the motor compartment out. See what comes out so you know how to treat to kill the pests, otherwise they will come right back.
HTH