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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Since this post is a year old I am probably too late with this reply, but here goes.
According to the Nissan service Manual, Noisy in park and neutral is oil level, or line pressure, or oil pump.
Dark, or black fluid with a burnt odour is worn friction material.
Varnished fluid, light to dark brown, and tacky, is under OR overfilling, or due to overheating.
The fluid capacity is 6 litres, but about 2 litres is in the torque converter, so does not come out when you drain it.
The Nissan Manual says to check the level with the engine running, and having fully warmed up the engine. Then move the gear lever through all positions, ending in park.
To use the HOT side of the dipstick, the oil must be over 50 degrees, so very hot indeed, too hot to touch. You nearly always want to use the cold side, but even that is for fluid over 30 degrees, hence warming the engine fully first.
It is very easy to overfill these gearboxes.
I refilled the oil 3 times to get it to run really clean.
I hope this helps.
Good luck