Skoda octavia gear problem

Hello. I have a problem with my car that when i have the speed between 67 and 85 km/h i hear a whisper. I can only hear it when i mentain the speed but not when i take my foot of the acceleration or when i press it down. Went to like 3 service shops and they told me its from the gear, something is loose or broken. And that it is not worth repairing and its not a big problem. But if i want to fix it, best solution is to replace it. I did want to change the oil in the gearbox and also add some additive in it special for this problem but everyone says that it wont fix anything. Anyone else had this problem? 

Thank you