I have a goodman 90% furnace... model gmvn0603 rev8, last year I cleaned the collector box coil. This heating season I notice a very strong "carbon-like" odor coming from the exhaust outside and it doesn't seem to be pushing much air out of the exhaust. I took the exhaust apart to check for air restriction... found none, took the inducer of to inspect operation and blower seemed to be working fine. Does it sound like collector box is restricted again. If so, why so soon and what causes this carbon build-up?