Don't think it's a wastegate rattle as it's not coming from the engine bay, it's coming from the rear of the car and definitely sounds like something to do with the exhaust. Either the flap inside or the exhaust vibrating against something. It's quite faint and only audible for a second or two as you pull away from a standstill but it does it every time I pull away so I've finally decided to try to solve the mystery.
I'll try the Golf tee mod to see if this changes anything, failing that I'll get the car up on a ramp to see if anything is moving. The car is under warranty but I find dealers are pretty useless at finding\dealing with rattles and creaks.