Hello X, Rick is offline so I thought I would pinch hit for him. If you accept, please click on one of Ricks replys.
I have this furnace and have serviced quite a few. The symptom you have is from a gasket behind the induction motor. We found that if too much stress was put on the assembly due to the vent pipe, it would open the gasket behind this motor. You may not have to even get a gasket.
It would involve removing the pvc pipe from the assembly ,pull motor, clean, run a bead of rtv silicone around gasket and snug back on. Test the pvc junction for stress. Sometimes the pipe does not line up well and guys will force it. This is what tweaks the gasket.