i recentelly face this problem in 06 cultus and overcome it by changing the 5th gear roller bearing,,,,,,,,,,,,,,,,it was giving noice when cold but it vanish as car heat up,,,,,,,,,,well i guess the primary cause of this problem is the wear in rooler bearing of 5th gear,,,,,,,,,,,,,,it magnifies in winter when you run it early morning when gear oil is as thick as grease,,,,,,,,,,,,,,,,another posible reason is that mostelly we never change the gear oil,,,,,,,,,,,,,the instruction manual says to change the gear oil after 40K ,,,,,,,,,,,,,,,,in my case my car is driven 90K+ and till now its running with original oil.,,,,,,,,,,,,,,it definitely need replacment as it become more thick because of carbon and also with steel mesh,,,,,,,,,,,,you will see lot of steel mesh deposited on the drain nut,,,,,,,,,,,,,,,,,,i use gaurd gear oil GL4 and i guess it is reliable,,,,,,,,,,,,,,