It's just a fan in a box - pulls air in - pushes air out. If there's a blockage, the motor has no air to move and no work to do - it will run faster and make more noise, certainly a higher pitch sound.
First fix the blockage in the airway and get the suction back (more likely to be on the inlet side but might possibly be on the outlet). Then listen for the "noise" again, if you think it's too noisy then the motor brushes or motor bearings may be starting to fail.
Good luck...
Peccavi, July 2009
