The "flexplate" is your flywheel in an automatic transmission equipped vehicle. It is a thin sheetmetal disc with a gear around the perimeter for starter motor engagement. Often the around the torque converter or crankshaft bolts, stress fractures turn into major cracks that, upon heavy torque (major acceleration) flex upon themselves and create the loud rackett we are referring to. If you stand on brake and give throttle (lots) this will occur without movement of vehicle. It also happens whenever you pull out (accelerate).
Also check motor mounts (front) and rear (transfercase) for movement. It may be your cooling fan hitting your fan shroud. Or could be your output shaft bearings in your transfer case, or your slip yoke.
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