Has the gearcase ever been off this motor? If so, it’s quite possible the copper water tube got slightly bent while reinstalling the gearcase and now it’s rubbing on the driveshaft. The tube is quite soft and bends easily so it might be contacting the shaft due to harmonics at certain RPM’s.
With the powerhead off, you can look right down the exhaust tube to see it. To straighten them, I’ve used a small wooden dowel and pried the tube closer to the housing than it is to the shaft. The tube will bend easily if it is burred or corroded and isn’t lubricated while trying to get it inside the rubber grommet on the water pump housing.
Here’s a water tube which was not quite worn all the way thru. If it did wear thru and leaked, the motor would have seriously overheated and have probably been lost.