Hi Dave, did you have the fan apart?? usually a 'rattle' means a loose nut/bolt, maybe a crack in the fan housing. does it quiet down when you touch the motor, or put a little pressure/torque on the motor or it's mountings??
Now [I'm nagging at ya
] quit wasting that 400* heat on the basement!!! and up the chimney, create some kind of bonnet that encloses as much of the stove 'box' as possible and run a fan through it. You will get as much vent temperature as you have now on half the fire, and a lot lower stack temp too. [OK, nag is over]
I know you need to have another project, right??