Still got warranty? Just take it back to the dealer. I worked at a Ford dealer and like '99-2004 Taurus and sables had an outside cowl leak where water would leak in around the cabin air filter and leak to the floor. The blower motor and resistor would get all rusty and stop working.
I'd start by looking at the cabin filter. The fan is below the filter and if the filter is hanging down or if something is stuck in it, it may be hitting the fan blades. The cabin filter, if equipped, will be the door at the base of the windshield, passenger side.