Has the trans been serviced?. They run a ZF 6HP26 sealed trans so there will be no dipstick or means to check levels. These are meant to be serviced at around 100000 ks and its not a cheap job. Its difficult to say what the noise could be but it's possible the clutches could be slipping slightly if its under a high load change.
If you decide to get the trans serviced then it's best to use the genuine ZF fluid. This includes a new oil pan as the filter is part of the pan. The service agents may have a better idea if they can hear the noise.