Transmission Knocking Sound?

I recently grabbed an 88 k1500 silverado $500 project. This guys wife drove it across a parking lot and slammed it into a Dumpster (on purpose), cuz he was talking with some Hotty's at a Wedding Reception,
she must be the jealous type. Anyway it crunched the front end in a little. I replaced inner/fenders and radiator bracket, hood , grill blah, blah blah. oh yeah, hit hard enough i had to straighten the rightside frame out back about 1 foot. All square now though.
It has the 5.7 and the 700 4spd automatic with +- 120Kmiles on it. Runs and drives good. Just started after driving on highway for 20 - 40 miles a knocking sound starts up only when in park or neutral. If i pull her into reverse or a foward gear the sound goes away. Pretty substancial knock too, I thought maybe a rod at first but since it subsides when in gear I was thinking Flywheel Bolt? I came off the highway pulled up to my buddies house, put it in park at idle speed and all you can hear is , sha-bam sha-bam., sha-bam. Pull it back in drive and nice and quiet.
Anyone seen or heard this b4?
-Forrest
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if he did that much frame damage, I'd guess either:
cracked bell housing cracked flywheel loose converter bolts loose bellhousing bolts
it's possible it has a broken crank, but that's a long shot

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