Take the hose out and cover the inlet hole with your flat hand - if you feel suction you will either have a blocked hose or more probably a split in the hose where the air is getting in - if you feel a poor level of suction the motor may not be turning as fast as it should or there's still a partial blockage in a filter area - I would guess a split hose given the things you say.
It's just a fan in a box with an inlet hole and an outlet hole - when no air can get in the motor runs faster (off load) and when air can get in it slows down a bit.
Good luck...
Peccavi, November 2009
