Quietest Backup Generator possiblly the Onan RS 30000

Has anyone tried researching a quiet backup LP, Natural gas, Diesel generator for emergency backup power? It seems nearly impossible to determine what manufacturers are claiming as silent. I went on Youtube and did some research. Even some of the manufacturer sites have these long 3 to 5 minute videos but dont even startup the generator to hear what it sounds like.
I did find one with an Onan RS 30000 Generator that was running and was actually fairly quiet compared to anything else that I could find, I could not find the DB spec at Onan site curiously it was not listed in the specification sheet. Its a 2007 video so not even sure if this unit is available, assuming from the size and 30000 designation its a 30KW model, it just sounds amazingly smooth even close up from the recording.