Had to fix my friends wrangler that was doing something similar at one point. One of the 'doors' that controls where airflow was going to had been kicked out of it's track by a 12 year olds foot. Best I could figure was the sound was a combo of air passing by the mostly closed trap and a loose vacume hose.
Just a thought!
the trap closest to the blower will cause a pretty distinct whirring sound if it sticks, you can carefully manipulate it with your hand with a little finese to open that trap to see if the noise goes away, if it does, check to make sure all of your vacume lines under the dash for the heat/ac are snug
hopefully its something simple and free
if you plug the blower up backwards it does it too
but i dont think you're having that particular problem
grinding noise, ignoring something stuck in the fan, sometimes the vacume acutators will make a popping or grinding sound if their traps are out of track, the traps sit in tracks similar to the way the fins in your ac vents are aligned and operated by a vacume plunger and metal guide lines