The post you're replying to is not about Harleys being silent in absolute sense but about their noise not standing out to drivers over the noise of other cars, car's own noise, and sound insulation.
Right, I haven't found that to be true at all. Some are louder than others (I assume they've been modded so?) but they're ~always quite loud and do not fade into the general background noise of the road.
Huh. Motorcycles are on a pretty short list of types of vehicles that startle or distract me with noise fairly often, along with emergency vehicles (popping on their sirens) and semi trucks (if they happen to rev to go up a hill or pass while right alongside me, or use a Jake Brake while fairly close).
Cars almost never do unless they've clearly got some severe problems with their mufflers, even though ordinary non emergency cars must represent something like 80% of vehicles I encounter on highways in my city. Even for the loud-bass types, the radius in which I can hear that and the volume is lower than typical "Hog"-alike motorcycles. Motorcycles are also the only non-emergency vehicles that sometimes sound really loud inside my house about 300 meters—and a bunch of trees—from the nearest highway (though that's mostly a couple of jerks who seem to like to race on that stretch of highway every so often, on what sound like racing-style bikes, not Harleys)
Motorcycles are loud outside the car, nobody argues that. Most cars have heavy sound insulation though. E.g. I can hear emergency sirens but just because of the different spectrum, if it had been engine sound at the same level I'd likely would not notice most of the time.
If I'm driving 30 or 40 mph then I will not hear a loud motorcycle directly next to me. Between my radio, my engine noise, my tire noise, and whatever insulation there is, I just won't hear it.
