It's not WG rattle, it's the sound of exhaust vacuum. It happens when the revs decrease suddenly and without resistance i.e. there is no throttle or engine load to sustain them.
WG rattle is a pig and is heard at the front of the car. It sound like a rattle snake with a sharp "chink" sound at the end of it indiacting that the WG pennies have finally sat back down.
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