Knocking sound, do I need Octane booster?
 

Its hardly audible, but I can hear it sometimes, 2005 Base boxster.
Some knocking sound in the background.
Our fuel in Costa Rica is 90 octane at its best.

Is octane booster a waste of money?
Can it damage anything? (If there is the slightest possibility of damaging anything, I wont use it)

Hi,

Octane booster is a waste of money. At best, it'll raise your octane a point or two, that means 0.1 or 0.2 octane (and the stuff costs the equivalent of about $27/gal). But, if as you say it's barely audible, I doubt you have a problem.

Pull the sparkplugs and read their insulator color. If whitish-gray, you're detonating, switch to a cooler plug. If a tobacco brown, you're OK. If black, you're running cold and fouling. This is a much better indicator than whatever you're hearing. Hope this helps...

Happy Motoring!... Jim'99

Jim is dead on with his advice.