MGWillia--
I have a 20KW Generac. I would describe it as perhaps a little louder than when a slab-mounted home Central Air compressor kicks on. (I know this is a relative term, since some AC units are louder than others)
Definitely not as loud as a lawn mower. You can definitely hear it, but it is not that bad.
Also, if you have a well and septic, I can tell you from personal experience it gets pretty gross around the house after a day or two without power.
Don't worry about offending your neighbors!