Auto trans clunking badly

Hello every one, I have a problem with my 1992 k10, after a fluid change my auto trans started clunking badly on neutral and park. Fluid is dexron and according to haynes capacity is 6 litres. It's weird because when I check the fluid it's reaching half of the dipstick and it's dark brown already so I guess 6 litres is far too much. But I tried dumping it down to the proper level and trans is still clunking.
I'm a bit confused here, anyone of you fine gentleman know what's going on? Would appreciate any suggestions.

Here are some videos of the car running
 

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