Heater fan noise

My Prius v fan for the heat started this low bass sound when it is turned up. It seems that it causes the vehicle to race! If I shut the heat fan off while parked, the vehicles takes a few seconds to return to "normal". Any ideas what might be going on? I am going to try to get to the dealer tomorrow, but concerned about driving it...since it is very cold here in Boston I need heat.

 

Update. .A Leaf! A Leaf! I've heard things in fans before but never like this, usually a flutter sound that you know something is hitting the blades. This was a deep base sound. If I had bet on this I woukd have lost. Too funny...now it is anyway.

 

I had changed the cabin filter thinking that it might be clogged; it has been in since last April when I bought the vehicle. It was dirty, put the new one in and still had the sound. I have no idea how the leaf bypassed the filter. I took it into the dealer, they too thought it was an odd sound, dropped the blower and inside was a leaf. We haven't had leaves in the Boston area since last Oct.......