Rythmic or random? It sounds like expanding or contracting exhaust/turbo which is normal unless it also sounds like there is an exhaust leak.
Was this after driving and then idling in the driveway, or not to long after starting?
See if it does it immediately when the car is started from cold. Or, when warm, let it idle for 5-15 minutes to stabilize temps, and see if it does it.
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