1999 Chevy Malibu Heater air flow

I have a 1999 Chevy Malibu and when I went out to my car this morning and turned the heat on I noticed that the air was not flowing through the vents are fast as it was even when I turned it all the way up there is not a whole lot of air flow, there is heat but not much air flow. Last Saturday I think I heard a little critter making some noise in the engine so I am thinking that it might have built a nest but I am not sure and I dont know how to go about checking this or how to fix this. If you have any advice please let me know, I live in Minnesota so it get pretty cold here and having good heat in the winter is pretty important. Thank you.