When you turn on fan,is air coming out of vents normal but not getting hot or cold.If no air is coming out,then blend door is shut,causing air to back up and maybe cause that noise,here are 2 links on how to replace
blend door and
blower motor,join the A.Z.site,at top right hand corner of page,it's so free,then you can use their repairs like me.I just had my evaporator replaced,it was clogged up,a mouse made a home in the box that holds it,air is suppose to go thru it,but it backed up and made a noise,had midas repair,cause freon had to be removed to replace part,it's located next to blower,your blend door flap is probably broken,just stays closed.