There are 2 spring-loaded catches which engage when the seat is at rest. The seat pivots on a shaft behind the bottom of the seat. In your case, there is a 1-2mm gap between the inner "C" part of the catch and the bar it secures to. When the seat is unoccupied, the tension of the frame is sufficient to press the bottom of this catch hard against the bar, but when someone is in the seat, the frame compresses slightly and this allows this catch to move about, producing the sound you hear
Unfortunately, this catch is embedded in the top of the seat but you can access the bar which is adjacent to the door edge, but behind a shaped plastic moulding which exends from the back of the car (visible when you open the door). You can experiment with some dense foam (I'd say start off with a piece 5mm wide and 10mm long and stick this on the forward edge of the receptacle that the seat catch engages. If you use a contact adhesive, you can easily take it off if you put too much on and have difficulty engaging the seat in the upright position
The root cause of this is because the seat tubular frame is a touch flimsy and alters shape when someone sits down.