Four speed transmission noise when cold

Just got this car from my dad (GLE model. I immediately noticed a noise when I drive it a mile it goes away. I took it to my personal mechanic and he did a drain and fill on the transmission. He noted that the fluid was like oil very dark, when he refiled it I immediately had issues driving in reverse but that went away when the fluid circulated. In the following weeks I had some slipping into first and that noise when I start to drive the car cold. So I took it to a transmission expert and he told me there is nothing wrong with the transmission, but it was a quart low on fluid. The following day I took it to get topped off. Good news no more slipping and the noise is less noticeable and only lasts about five minutes only when the engine is cold. I have tried self diagnosing by driving with the noise then slamming the car in neutral and even killing the car, but the noise goes away but I can hear it a little when revving the engine in neutral, it seems the noise is only when I accelerate. Also I can get it to make the noise if I am in gear and have my foot on the brakes and gas at the same time. I can also hear it a little in neutral. Note: The transmission has a lifetime permanent no serviceable filter that you have to remove the transmission to replace it. I am planning to do another drain and fill, contemplating using Lucus transmission fix? I have a video below, please help.

Update: Okay, so this morning I put it into drive and started driving and there was the noise so I hit the gas then put it in neutral and hit the gas, the noise was less obvious but it was still present but much quieter. So I put it in park and hit the gas, and the noise was there but very faint. Seems like the car driving makes it louder, but when the engine is warm the noise goes away completely.