It's not 50% unless you live in a tight-knit ricer neighborhood. It's not a trivial option to customize a car to make it louder; it takes a bit of effort.
What is true that we tend to notice annoyances more than non-annoyances, and that the car may be louder because the driver want to accelerate or velocitate more than they should, and noise is a side effect. Or the stereo is louder than it should be, not because they want you to hear it, but because they want to hear it and they don't care about you.
My observations are a bit different. I live in a populous suburb (150k residents?) north of Denver. Truck culture is very prominent here and mainly consists of raising a vehicle and making it louder - mostly because they do want to attract attention. My 17 year old neighbor modified his Subaru so that it can (literally) be heard from miles away - and when he simply turns the car on can be heard clear across the entire neighborhood until the engine warms up and throttles down.
I'm not so much annoyed as I'm curious as to how all of this noise affects the environment at large (people's sleep quality and mood, birds migrating, insects mating etc.)
It can actually be extremely trivial and cheap by just drilling holes in your muffler (yes, I've seen this). Or by installing a glasspack for ~$20. This is what everyone gets annoyed at, usually.
What is true that we tend to notice annoyances more than non-annoyances, and that the car may be louder because the driver want to accelerate or velocitate more than they should, and noise is a side effect. Or the stereo is louder than it should be, not because they want you to hear it, but because they want to hear it and they don't care about you.