Bakesy, if you have tried a higher octane fuel and it seems to cure the rattles, stick with it.
Even if it doesn't, stay with it while you find the cure - and after!.
I have run my G13A on nothing but BP Ultimate 98 since that fuel was put on the market, I use 95 Premium only when I have to and avoid 92 octane Regular completely. I had to fill up with 92 on a bush trip last year, within 15kms the engine was pinging its heart out under any sort of load but it all returned to normal once I found some 98. The G13A has a slightly longer stroke than the G13B but they are otherwise identical, and both love higher octane fuel.
As da13ro said, it is completely safe to use, and the extra cost per litre actually works out cheaper as the slightly better power means you use less fuel. Okay, it's not a huge power boost but let's face it, with a Zook any boost is worth it.
Incidentally, my engine is completely stock apart from extractors and a modified air intake (a scoop behind the radiator grille - I don't do water crossings!)
Cheers - Longbaser.
An economist is an expert who will know tomorrow why the things he predicted yesterday didn't happen today. ― Laurence J. Peter