Maybe new ones suddenly got quiet, but I doubt it. I wound up getting a 12KW Generac unit that was about 5 years old from a neighbor because it quit during an outage and the service company told him it wasn't worth fixing. It looked almost like new, so I thought I'd see if maybe I could fix it and use it. It started up and ran, so I heard what it sounded like. And it sure wasn't like any idling car, except maybe one with a broken muffler. More like a loud garden tractor, which kind of makes sense, since it's a similar ~25hp engine.
Here's a youtube of one if you don't believe me:
Make sure you listen until about 25 secs in, when it actually starts. Before that there is something in the background that does sound more like a car.
Let me know what you think after you watch the video.
I've also been
One big difference there you're talking about an engine that's an order of magnitude smaller than one in a whole house type. The quietest ones are the inverter type, where they sythensize the AC waveform. That decouples the frequency from the engine speed. At low loads, the engine can run at lower RPMS instead of full speed.