Gearbox noise 4 gear switches

Hello, my /8er has a problem with the gearbox. When it is cold one hears a relatively loud noise from the direction of the gearbox. If you step out of the clutch the noise is removed as soon as you connect again – noise also there again. However, if the car has set a distance of about 40 – 50 kilometers back, the noise is hard to hear. Gear oil I have therefore already changed and also a chic additive with in tipped. A screwdriver colleague said that the main shaft in the Get probably had a problem. Did anyone ever have the problem here, or how would you proceed now? Get out the gearbox and overtake it, if that is at all possible or immediately a replacement gearbox in?