Possibility that the pump gear has loosened on shaft and moved forward so teeth are only engaging a portion of the nylon tooth width.
What is the serial number of this 331? The 331 covered several years of production had several design changes during production. One of major changes was introduced when 331D came out, the D series and later use a variable displacement, load sense pump. Earlier versions had 3 section gear pump. All versions used mechanical linkage on blade control, meaning that the are rest switch did not affect this function so diagnose problem using the blade function.
Now if it is a D series or later, has piston pump, and drive coupler is fine and no blade function, that narrows it down to problem in load sense circuit. When engine running with all functions in neutral position the pump is at minimal stoke moving just enough fluid to maintain the pilot pressure at the joystick. When any valve is activated the change is pressure and flow is sensed through the load sense line from valve to pump allowing the pump to move to on stoke sending fluid to the function.
Summary: If all functions are inop including blade and machine has a gear pump only commonality to all functions is the coupler. If it has the the piston pump the pump coupler or load sense circuit could be the issue.
