Accumulated dirt on the fan blades
Hi, I am not sure what kind of system you have but I can help you and tell you why this happens. This is the blower motor, and you also have a return air side on the front of the unit. The air is pulled back through the motor, that the filter doesn't catch, and collects on the blades. This takes place over time. If this is a window or wall unit, you may be able to turn off power and reach in with a brush of some kind to break loose this dirt. What happens over time is the dirt will collect on the windings of the motor causing it to overheat, so you want to clean this the best that you can. Even if you had a small air compressor it would be hard to get the dirt off the blades, but would be good to blow out the loose dirt. If this is a central air unit, you can remove 2 screws in front and slide the assembly out and clean the blades this way. So, find a way that is easy for you and clean as best as you can. Be sure you change your air filter on a regular basis. Please don't forget to rate me on this as I know you will be kind. Please keep me posted if you need my help.
A/C, & Heating Contractor