It's not that it's louder, it's an unusual tone. People are used to the typical purr of a nicely muffled Briggs, Tecumseh, or Honda 4-stroke mower, but the high-pitched crackle of a DuraForce I could see being somewhat annoying if you weren't into small engines the way we are.
Back in the day, my dad used to have a little Suzuki motorcycle with a 2-stroke engine, and it'd wake my neighbor up every morning when he'd head off to work. It was super quiet, but that tinny "rinng dinng dinng" sound was something unusual compared to most other bikes. If he'd have rode a Harley, they'd probably never notice. Sounds strange, but certain sounds just get to a person more than others.