On long highway trips ( + 500 km ) , I need earplugs  in my .2 RS   

As Dave stated, it is more general noise , like tires and so then exhaust noise that gets you after a while . The car has more of a resonance inside due to less insulation . 

I did not need any in my .1 RS 


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 964 Carrera 4 --  997.2 C2S , -20mm -- 991.2 GT3 RS