I have had the transmission replaced and this problem is recurring.
After the vehicle has well and truly heated up, noises and grinding occur in low gear...one it shifts up a gear the noise goes away. It has a transmission cooler fitted but it is getting very frustrating as this will be the 3rd replacement and the same thing is happening after a period of time..also at 100ks/hr there is sometimes a surge in revs....
Are you sure its not a part of the engine/trans/exhaust/diff hitting the body. This will give a grinding sound that goes away once the pedal is lifted a bit. Re surging at 100kph that could be 4th gear locking and unlocking...maybe the auto is getting wrong instructions from the ECU....speed sensor?
I'm having pretty much the same issues with my transmission, still the original trans with 175000 on it. The surging revs is wht got me stumped. I was gonna go get a good service and take it from there. Hoping its not gonna need heaps of major work...then there is always a manual conversion