Performed heating tune up service on heat pump system checking all electrical components, refrigerant balance, unit start up, operation, defrost, shutdown. Customer noted a scratching noise as unit shuts off in heating mode. This is caused by a couple of different things happening upon system shutdown. First, expansion and contraction of the metal in the unit as it begins to cool off, second, a slight refrigerant noise can be heard that sounds similar to flowing water, third, the blower wheel rotation is still audible for about a minute as the blower begins to slow down after shutoff. All three of these events together appear to cause a slight scratching noise at the air handler and resonates through to the ductwork above the unit.