[gclions]

Have someone else cold start the car with you at the back, listen to the mufflers. One or both of the exhaust flaps that open as you go to wide open throttle may be vibrating on startup. I would start there, if that's not it let me know.

Also, I don't think it's a waste gate issue, one because of their plunger type design, and two you aren't at boost on startup so they should be completely shut.