It very well may be a dead animal in your system. It could be in the actual duct line or could have been disintegrated and went into your system as fine pieces. You need to first get your ac guy to check things out to make sure something is not stuck somewhere in the actual ac unit. Then have a cleaning company who specializes in duct cleaning such as servpro or any other national clean up and restoration company to check out the actual duct lines running from the ac system. It's very likely a rat chewed his way into the duct and died. Before you call anyone to come and check it out you can get into your crawl space or attic(wherever your lines are) and check to see if any holes are in the line. This could help narrow the problem down. Duct cleaning is very expensive so good luck.