I know this sounds odd but I have found Forte Injector cleaner can fix diesel knock. I can only presume it is over or under fuelling that causes it on some vehicles.
I have had a couple of instances recently where a vehicle sounding as rough as sounded sweet after a dose of the above product on a 1/4 of a tank of fuel and leaving it to sit for 24 hours after a short run once dosed. After 24 hours or even longer if possible take it for a decent run and then fill up with premium diesel and a 300ml shot of two stroke oil.
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