pond pump humming, but water keeps flowing
Pump motors usually spin fairly easily. My guess is the bearing seals have failed and the extra drag is causing the motor to buzz or hum.
It is possible you can disable and get to the bearings on the shaft. It is normally the output shaft bearing that fails.
A high quality double sealed bearing for a small pump like that will normally be only a few dollars, and you do want a type ZZ or 2RS with most brand  zz= double sealed 2RS is 2 rubber seal , so look for a number like 188zz or 608-2RS  these are examples of the 2 smallest motor bearings I have on the shelf that I use in motor applications both are well under 1$ each and I usually purchase 10 at a time because they are cheaper but you want the rubber sealed.
Be careful on disassembly as there may be a seal just like an oil seal on a car crank shaft that is usually able to be re used if careful.
But it’s well worth it to repair a nice motor for 2$  and an hours time that may save over 100$.