If this is what I think it is, I've got this too.
Replaced my blower motor recently as well with a used unit from an '88 SSE that a family friend was parting out (2nd tranny failure). The noise was there before *and* after replacing the motor, so I don't think that'* it.
I get the noise randomly, but it also tends to happen upon starting up the car. A guess would be the door/flap that directs air to the foot vents or upper vents, etc. The heater/fan/AC doesn't have to be on for this noise to be there.