Used to have something similar on my 911s happen when the motor mounts got week. happened more off the line and into 2nd gear. once car caught up to itself it didnt happen. More load when going from standstill so maybe more load going up hill in the Z. My thud was in the back end but thats where the motor was. if your engine is flexing under that initial load, you might hear the thud - or if it causes things to twist then maybe it is twisting the exhaust pipes?
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