I have a Carrier Weathermaker 9300TS furnace, about 5 years old. Earlier this year I had to have the blower motor replaced. Within the last week or so when the furnace comes on, the blower motor starts up first, I think that is to clear the fumes from the furnace, then right before the unit actually starts up to heat and the motor picks up speed, I hear a metal knock noise. Any suggestion what might be causing this? I read on this site someone had the same situation but it happened when the furnace is shutting down. Since it cost me almost $600 for a repairman to come out on the blower motor, I hate to call him again on this if it is something simple. If it is any help, I have a ranch style house, the air comes in from the bottom, goes out the top of the furnace and duct work runs alongside the furnace taking the hot air back down under the house to the heat vents.

Sounds to me like the vent motor is rubbing on the housing at a certain speed.