We still have a generator guru or two who drop by here, but most have moved on. Sadly, largely because owners didn't provide detailed make/model/spec code (all important if it's ONAN, don't know about others...).
Notwithstanding, SmokStak.Com is all about engines, tractors, generators. They have a Generac section and you may get better play there.
Agree with Drew, this could easily be a touch of overspeed. Wonder if a linkage got sticky or a spring came off.
Every manual will tell you to have a Frequency Meter to set up the RPM (the governor, etc.) But if it's a simple fixed RPM generator: RPM, Voltage, and Frequency are locked together. In other words, an old 1800 RPM ONAN, running 120VAC (on a common meter we all have) will also be 60 hZ and 1800 RPM.
But you have electronics managing your Generac, so checking stuff probably won't be that simple. Remember too, if the electronics somehow allow it to run over voltage, or you bypass something to do that, the higher voltage puts all the "customer" appliances and stuff at risk.