vibrations

Don't know why you would be operating your engine at 4400 rpm, in flight  but sounds more like a propeller balance issue or if airspeed/attitude the same each time, possibly a wheel rotating.

It could be the prop or the gearbox. Need more info on engine time and maint. More info on the prop. Did you check the pitch on each blade it it's  a ground adjustable and has your prop been balanced.

I overhauled and balanced the propeller. In my opinion if there was a problem with the propeller then the vibrations would have been constant. This is a random phenomenon that occurs once every 10 hours of flight. The engine itself has gone through 1200 hours of flight in 10 years. I performed all maintenance according to the manufacturer's instructions.

When cruising, this is the recommended RPM according to the manufacturer's instructions

An important point: when the vibrations appear, I open the throttle to the maximum. The phenomenon disappears immediately. Even when I return to cruising at 4400 rpm