Additionally: Do you also think it would be a good idea to use 98RON during summer months?

Yes

http://newsroom.aaa.com/2013/06/what-is-the-difference-between-summer-and-winter-blend-gasoline/

http://www.atlasoil.com/Blog/Whats-the-Difference-Between-Winter-Blend-and-Summer-Blend-Gasoline

_________________________________________________________________
Speed is subjective to only how far you keep your right foot down..

Generally the rattling sound you here is called 'knocking' & is timing related.
I guess though if you have the world's dirtiest air filter, crappy plug leads &
very worn / damaged / fouled spark plugs the same situation can occur

_________________________________________________________________
Speed is subjective to only how far you keep your right foot down..

In addition to Madaz, an additional reason can be leaning out of the mixture. So some more causes could be (not limited to) air leak, restricted fuel supply somewhere (e.g rail pressure or blocked injector), failing O2 or MAF/MAP sensor (leaning out the computed mixture but not enough to throw a fault).

Other causes: - built-up deposits in cylinder, wrong heat range plugs.

One remote thought is an overheating cylinder through (say) blocked cooling passages.

Noticing under what conditions it pings can help. Under load or cruise? High revs or low? Hot or cold? etc.

It might be a few things together.

Would suggest NOT disguising it with an octane boost... yet.

Do some basic checks (ignition system-plugs/leads/dizzy/timing, fuel filter, air filter/leaks check, fuel system check), fuel system/injector cleaner, clean sensors then listen for initiating conditions or see if it's better.

Let us know how it goes!

Patrick

The BA had an old-style ignition timing arrangement with a distributor that could be rotated by undoing a bolt.