Try with a modern open wheeler like the formula 3.5, gp2 or formula masters. From memory, driving in those with a flat spot produces the most violent vibration/rattling in your ffb wheel. It should be with a g25 too. If you're not able to notice this then it would suggest something else maybe wrong with your ffb setup muting the ffb details in rf2. Also visually check you have flatspotted a front tyre(s) which is easy to do on an open wheeler ofc.