"Can you confirm the overspeed acoustically?"
Yes. It is distinctively audible and correlates with the tach reading.
"I think cavitation is a marine propeller phenomenon in which the water vaporizes. This cannot happen in air, as far as I know."
Wrong term, I guess. My thoughts are that the propeller air foil is stalling, briefly, induced by turbulent air flow around the engine and muffler.
does sound like fan surge, where the flow rate through the fan becomes
unstable. How many times has this occurred, only once? What is the pressure altitude you are flying at when this occurs?"
My thoughts, exactly. This phenomena occurs every time I fly, therefore varying conditions. It is always during acceleration during take off (~25-35 mph) and occasionally early in climb out. I suspect the problem may be worse with heavy loads and warmer air temps although it occurs with lighter loads and cold air.
I have calculated the propeller speed, although I do not have the info handy, and it is well below Mach 1.
I have a 3 blade Warp Drive that I will install soon and will give an update on performance.
Thank you all for your input . Any further input is appreciated.