Heat Pumps and Electric Heating - Outdoor Air Handler Has Bad Smell

I have a fairly new (last 5 years) Goodman all in one heat pump and air handler on a slab outside my house. In the past couple of weeks we have noticed a foul odor- musty/ earthy from the vents throughout the house. Any suggestions on what I could do to rid the system of the odor. I was going to try and open up the unit and see if leaves or something had gotten inside. Is this something I can take care of or should I suck it up and call a pro?

Package units are fairly easy to clean, the truth known is that a HO whom is diligent and takes his time will probably do a more thorough job than most pros, who are always in a huge hurry. If I were you I would call in a Pro this first time and watch closely at what he does (get him to sell you some coil cleaner that way you can perform the annual cleaning yourself from now on if you so desire). If your unit is five years old and never had the coils cleaned you probably have some real nasty stuff growing on the coil surfaces which will produce some odd smells and also make your unit less energy efficient. Git er dun. Once the coil cleaning is done if you still have odor you have a ductwork problem either a return duct is loose or flex connection at the unit which is allowing crawlspace smell into your air.