Could clogged exhaust cause codes 13/33 on my furnace, and how do I check and clear?
To me it sounds like a motor lubrication issue, your fan motor that sounds like it is struggling is probably the issue. Check for lube ports on the fan, if there is a port on the motor case both front and back try a few drops of motor oil in the ports, if there are no ports the bearings may be zz or double sealed and normally need to be replaced, in a pinch one of the shields could be pulled and fresh lube added in the form of a light grease as oil will just drip away if not designed for oil and grease is used.  The last type is an oil light bearing. These are not usually used on quality fans but possible the bearing is a sleeve that wicks oil from a wadding adding oil until the wadding is saturated will usually give new life to the bearing (these are usually found on low load fans like bathroom and low cost window ac units). There is probably a cane switch or a pressure switch that monitors the flow and may be triggering the fault code, oiling may help the fan to produce full speed/flow and solve your problem.