How can I be sure of this? The pump still runs and shows rpm at max. Could there be a blockage somehow?
because rattling noise is a strong indication of proller being either detached or shifted downwards from the axle. This would cause the prollered either collide with the finned side of the copper plate on the water side or propeller hausing. Since since you have flow and return hoses at roughly the same temperature further supports this premise.
Sometimes even axle itself can have a slag that can make it oscillate up and down or even to sides.
RPM will still be shown as the motor spins and and rpm reading is done with hall effect sensor which is mounted near magnetic core (nowhere near propeller). If you're interested in...