Do you get the noise with the pedal pushed down and in gear?
If so the trans has no moving parts at that time. If this is the case the mechanic is dishonest.
A transmission makes noise when the pedal is up, either in neutral or when driving.

Cheers, Charlie
If It ain't broke, Modify it!
87 Montero turbo Converted back in Spring1989
95 Montero SR 3.8 DOHC Only one?
93 Pajero 3 door 6G75 Mivec with paddle shifted 5 speed
Then a Gen2 SR with full coil independent suspension.