The mechanism is essentially airtight. This sound indicates that the vacuum seal has been broken and air has rushed back into the bottle.
Air Entry Technique to Break the Vacuum Seal
Hold the bottle upright on a stable surface. This usually indicates the pump head is jammed or the internal spring is stuck against the housing.
Push down firmly on the pump until you hear a distinct "pop" or hiss. When you press the top, you are compressing air and liquid out the nozzle.
Air Entry Technique to Break the Vacuum Seal
Remove the bottle, dry it thoroughly, and attempt to pump normally. The vacuum seal is strong, and the bottle walls are concave.
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