Not even sure where to start

Could be air locked, or a blockage at the impeller. Make sure you safely disconnect the pump from its power source before trouble shooting...

Most likely you have hair ball or piece or string tangled up around the propeller. Remove lid, lift the pump and see if propeller spins freely. If propeller is clean than you may have to replace pump.

Back to you

Milo

Do you mean it hums 24/7 or all the time when it is running