heater/fan dysfunction

My heater fan (at first) worked fine on all settings, then started working
intermittently on the highest level.  Funny sounds at that level then, too.  It
eventually stopped working altogether.   The other levels were fine for a while,  
then the next highest level did the same thing.   Same for the last ones, as well.  
The defogger doesn't work now either.  Trace amounts of heat come through
the vents.  The engine isn't overheating.  It's a 2004 Chevy Cavalier.   Cause???