This thread is E-Tron GT specific and only for the intermittent humming between 38-43 mph.
Good news and a warning.
After the latest software campaigns
93P9 and 27K2, (actually 5 updates in total), one will change how and when the front motor operates.
The software update results in improvements to the distribution of the drive force to the front and rear axles and to the deactivation of the drive at the front axle. This leads to increased efficiency in the drive system and to reduced energy consumption, and therefore to an improved range. In addition to this, the likelihood of a potential decrease in battery performance over time is reduced.
The good news is that it will almost totally eliminate this issue.
The bad news, it's possibly also a workaround, by shutting down the front motor at those speeds to remedy the vibration.
If it was hard to convince the master mechanic in the past, be aware that it will be a lot harder to simulate the issue after these updates are applied. I feel like a ninja, still capable of reproducing the buzzing noise in a tiny operating window. But I have to admit, it's mostly fixed, and if you don't know what you're looking for you'll never know.