4x4 gearbox 'clunks'

The more I drive my 4x4 the more I notice that it makes a clunk noise when selecting some of the gears, in particular 3rd. Its not all the time, only when I hurry to change, so is it me not hitting the gate properly or something more sinister? It slips into gear nice and smoothly it just makes the metallic clunk sound. The gearbox has always been the one thing I dont really like about the octavia, seems far too vague and the spring loaded stick doesnt suit. Maybe its just taking its revenge on me for slating it! :confused: