It time to change your air filter on your furnace 

   Your force air furnace needs to stay clean. The A Coil  ( evaporator coil) needs to be free of debris so air can pass through allowing to cool or heat the house. If the coil gets clogged it is a very expensive to have clean, because the furnace in many cases needs to be remove to have access to the under side of the coil.

The pleaded filter that gets dirty make the blower fan work harder, which could shorten the life of the furnace. It could also affect the efficiency of the furnace.  I recommend the fiberglass type filter

Pleaded Filter

Fiberglass Filter

The filter recommend every 30 days, I find every two to three months work if the house not that dusty.