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my Clubsport used to clunk a little, changed the oil and put in the additive - sorted.
Not sure if the additive will do much. Lsd additive mainly tries to stop chattering so your centre lasts longer. I'd check your driveshafts like someone above mentioned. Could be a bad bearing or backlash set wrong too. But check everything else out first. You would be lucky to fix a clunk with new oil.
yeah I was surprised it did tbh, the clunk though it's such a pita as it can be a few things. gearbox pressure bs, this that and the other - the additive definitely stops some of them.
Hey it's definitely worth a go. Oil costs bugger all so if it doesn't work no great loss.