Update on that topic:

The metallic sound I was hearing (like a ding-ding-ding noise) coming from the bottom of the car was not from the transmission, it was the 'pinging' sound of predetonation (because of the working knock sensors), the sound was coming from the exhaust, not the automatic transmission. I cured it by cleaning the carbon deposits with Seafoam engine treatment, added the fluid in the intake with the use of the vacuum of the small hose from the pcv valve with the engine reving at 4k while the engine was already well warmed up. That produced a massive stinky white cloud. After the car had drunk the full bottle, replug the pcv hose, I drove on the highway at 4-5k rpm for 20 minutes. It cleared up the pinging sound.