Check the ties at the back of the rear seats (the ones used to stop the seats moving around when they are folded down). The ties have metal bits at the end, which are supposed to slot into little sleeves when the ties are not in use.
When I first got my CRV, I had a metalic tapping noise too, and it turned out to be that one of the metal bits was not in its sleeve, and was knocking against the back of the seat.
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