Yes, Growling , .
My F&P Neo conversion used to growl just when it started to produce a certain voltage /Amps. , and then became silent . On the slow down ,it came in at the same Voltage/Amperage ,then silent til stop.
I looked at my meters and knew when this happened, so from then onwards while in the field or where I could hear it ,I knew what it was producing --sort of good.
Bruce