What is decibel level and can this be converted to propane?

I metered it last year and it was 67 Decibel (Using Sensatic Mobil 1 Oil) and I am not sure about conversion part of your question but everything its possible this days, just look for the kit in the internet, I have seen it!

No decibel rating, noise level is a variable, depending on load increasing RPM's. No propane conversion kit available from Generac.

Generac doesn't test the sound level of its portable generators because there is no industry standard, and the results could be misleading. However, the sound level of our portable generators is comparable to many portable generators on the market. Generac does not offer any accessories to convert this unit to propane fuel, and doesn't recommend such a conversion for performance and safety reasons. We do offer an LP3250 portable generator that has been designed specifically to run with a 20 or 30 lb tank of LP fuel. If you are considering this generator for home backup power, however, we would recommend considering a Generac permanently installed home backup generator. These units run on LP fuel, start and stop automatically, and are connected directly to your home's electrical system.