1999 323 Convertible - funny knocking sound

Hi, I am experiencing a funny knocking sound - it sounds like 2 soft knocks on a door at intervals of 15 seconds i.e. 2 knocks and then 15
seconds silence. It doesn't happen all the time and is most noticeable when idling- maybe because there is less noise at this time.
I know this sounds vague but any ideas are appreciated.
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