Here's a noise test done on the honda generator by some university students. It seems national parks standard are ratings done at 50ft, and need to be less than 60db on the A-weigted rating scale to comply. The A-rating scale makes the decibels seem like less than they really are because it's only for lower frequency noises. The actual db of the generator are much higher as you can see in their graphs. http://www.me.ua.edu/stal/Pages/PDF/HondaNoiseTests.pdf
To give you perspective, for the honda eu2000i you are looking at about 67db on the non-weighted measurement, and 56db on the weighted measurement for 50feet. That's like someone have a full conversation right outside your window while you're trying to sleep. If your neighbors are closer than that, it will be much louder.
Even with a honda eu2000i, which we use at work. They are still really loud up close. I'd probably put one in an enclosure with some dynamat if you want to use it in a park.