Again unless you were to do an in depth measurements analysis of the frequency it would be a pure guess were the source was coming from exactly, bearings cogs main shaft etc, but if you were able to measure its frequency response then that is were I would probably start.
Then you could start apply masses etc, this is extremely time consuming thou.
Look underneath vehicles especially convertibles you will find quite a few bolted on masses, they are meant to be hidden or blended in to its surroundings, without the tin top the vehicle changes a lot so hence the countermeasures used.
These factors are supposedly calculated at a design stage, but in the real world nothing is clear cut and perfect.
Manufacturers will make a judgement whether the average driver will pick up these noises at a development stage, it is then decided whether it warrants additional money and development time to correct the
problem, which in turn increases cost on the vehicle.
Even with what I do it took me sometime to pick up the gear whine, although boom was pretty evident from the test drive but I can live with that as was road noise although I have done some isolation changes and improvements to the rear paths of the vehicle to cabin.
On the last service I had my gearbox oil renewed, the car is at 60000 miles. I have a VW garage do my servicing as I find them far more detailed about servicing, I noticed to be honest a slightly slicker gear change but no real noise reduction.
The garage recorded that the level was down by 500ml which was not a great deal but if my mind serves me correct you would have to ask for the gearbox oil to be checked as it does not always come up to check on servicing, please correct if I have that wrong on this.
