fan noise

I have a 2003 Pontiac Grand AM.
lately when I turn the heat or the air on the fan makes some crackling noise.
when the engine is off and the fan on there is no noise.
does anyone have an idea what may cause it .
Thanks

2003 should still be convered under warranty??

be careful before you take it in for warranty. If it is just leaves or debris in the heater box, it will cost you about $80 for your dealer to remove them, as removing leaves isn't covered under warranty