Having driven most plug-ins on the market, highway noise is mostly a factor of tire design and soundproofing. With ICE, unless gearing is very wrong or you're in a Mitsubishi Mirage, often you won't hear the drivetrain. So EV's appear noisier at speed in contrast to their lower-speed silence. Below 40 km/h, electrics offer a silence no ICE can approach, but once up to speed, air and tire roar factor in and you go from zero noise to 67 dba, making EV's appear noisier than ICE. With the latter, drivetrain noise is prevalent from idle to city speeds, and at speed mechanical noises are replaced by road noise, so less contrast.