Diagnose a pinging/ clicking behind the center dash stack?

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I have noticed a faint clicking about every 10 seconds maybe near the firewall on my 02 S60 T5. I can only hear it when the engine is idling and the radio is off. I thought that it might be a heat shield at first cooling off while I was stopped. Could this be something else in the exhaust or maybe a heater core? I am happy to try to describe in more detail if you might have an idea.

Thanks.