MY 2004 Buick Century is making a hissing squealy noise on 2004 Buick Century

For a while when I shut off my car it would make a hissing type squealing noise. Now it started to happen when I drive also; but it would stop hissing when I put the fan on. Sometimes when i change the temp controls it will stop but not all the time. It only stops ALL the time when i put on the fan. What could this be?

Asked by for the 2004 Buick Century