I got a wierd one....someone may have seen this before.
I checked out this furance everything worked fine but when the main blower turned on a real bad noise came from the squirrel cage like it was hitting the sides. Opened it up and found that the squirrel cage moving back and forth, telling me the bearings on the motor had gone out. The squirrel cage was wobbling enough vibrating like crazy making it sound like it was about to blow out the back. So I proceeded to replace the motor, making sure to inspect the squirrel cage for damage, cracks, run out, bent, dirt build up etc.... I found the wheel to be in good shape so i centered everything up, put it back into the furnace cranked it up and wallla everything was blowing again, no noise, no significant noticable vibration. Keep in mind, during inspection I made sure that all the "weights" were still in place none appeared to be missing nor were in the bottom of the volute, I also cleaned of any significant dirt.
I get a call 2 weeks later and they say they just heard a vibration that hadnt been there before. Opening the furnace and running the blower it starts out smooth but when it gets up to full speed the slight vibration is amplified through the duct work. So I take it out of the furnace all together and run it by itself with the wheel on it, still feeling a small vibration. So I proceed to remove the squirrel wheel and run the motor by itself and I still feel a slight vibration....hmmmm. Is the wheel bad and its taking out the bearings in the motor? Did i get a defective motor? Kind of scratching my head. My instinct tells me to change motor and squirrel cage both.... anyone run into this before?