Transmission fluid disappears over time?

[Taiwwa]

Car has 130k miles.

 

Car was making loud noises so I changed transmission fluid. In videos it had made it seem like 5 point something liters were needed. I ended up putting in six liters. Drove it some more.

 

Still noisy. Not all gears, but certain ratios especially low speeds going up hill were very noisy.

 

So the other day I opened it up again and put in another liter.

 

Now it works much better. It may still need some more because I can hear a bit of noise at certain ratios so I’ll put some more in ends oil change.

 

But initially I only put in six liters because that was what the videos I saw advised, and because that was what I roughly measured in removing the used fluid. In fact, I thought I may have slightly overfilled it at the time.

 

So what gives? Could the transmission fluid have been vaporized over that time? I thought it was a self contained system.

 

I think that it is a good idea to change the fluid at 100k miles, not 150k or 250k.

[plus 3 golfer]

Transmission fluid doesn't "vaporize".  My guess is you are hearing the infamous "thumping" TSB sounds associated with bad bearings on the transfer shaft.  The transfer shaft will ultimately bore a hole in the transmission case and thus fluid can leak from the engine side of the transmission case into the damper housing case and eventually onto the belly cover.  See the latest "thumping" TSB. https://static.nhtsa.gov/odi/tsbs/2018/MC-10148717-9999.pdf