[piecesparts]

No, not necessarily at the start up. A cracked flexplate gets that way, due to the ballooning of the torque convertor. Ballooning is expanding of the convertor due to pressure build up in it at different RPMs (this is by it's design). That means the torque converter will swell longetudinally along the input shaft as the motor runs.
When you start the engine, there is no load on the convertor and when you put the truck into a gear, the loading starts, which means the convertor is now starting to work harder. If the flexplate is cracked at the torque convertor bolt holes or at it's center bolt holes it can make noises as the convertor finds it's new position to the load that is exerted.

To find this, you will have to get under the vehicle and put a pry bar against the flexplate and see if you can see cracks as you flex it. If the holes are on the center bolts, then you may not see them until the tranny is loosened and backed away from the motor. I have repalced the flexplates on all of my trucks with a heavy duty SFI rated flexplate. That way I get away from the situation that I just described. I leaarned this the hard way by having to replace a couple and that takes time.