Whatever it was, it wasn't INSIDE your torque converter, else you'd be scrounging up a used tranny by now. Small pebble bouncing around inside the bellhousing, pretty good chance.

Towing the car at basically zero mph wouldn't even get the crankshaft to turn - that fluid coupling needs some decent rpm to transmit power in either direction. The car just can't be in Park for the 10-foot tow onto the flatbed, and shouldn't be in Reverse, but neutral or Drive is fine. Recall that since about 1960 cars with automatics can't be push-started... although I'd like to see Myth Busters or some other TV junkyard warriors see if a push- or pull-start at 60+ mph would work.

Could be that just the steep angle of the tilted bed on the flatbed was enough to dislodge whatever was banging around in there.

So when you get that cracked pipe repaired, let us know what happens to your gas mileage!
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Thank goodness we don't get all the government we pay for. -- Wiley Post