Could be the Clutch band for that gear is wearing, but not likely as the others would go as well. It's not the s5 solenoid as this controls most of the changes as well. Don't quote me but as there are a few solenoids in the gearbox, it could be one of those. Not as expensive as the S5, but if you ring a few Auto guys, they should be able to tell you.
Also check the oil for any metal fragments, then let us know.
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If there's commotion coming from the passenger seat, and nothing but earth going round and round outside the windows,.......then everything is not as it seems!
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