if you suspect mold you should also
look inside the supply plenum.
90% of the mold I find is inside
this plenum.
once it is established in the plenum,
it takes a while for it to spread into
ducts.
taking off a duct & looking inside the
supply would be where I'd start.

best of luck

Cleaning anywhere you find mold is certainly good but all mold is not created equal. Some is bad and some is worse then some is just ugly. In addition to cleaning, useing pleated filters THAT FIT WELL is the most effective measure you can take for your health and unit maintainance. Contrary to "they said and I heard" , changing filters every 30 days is usualy wasted time and money. Every household is different when it comes to filter change intervals. A single person or couple that work and play in a clean inviroment are probably fine changing 2 or 3 times per year. A family of 4 that gardens,plays baseball,owns a woodworking shop and pets could be over due in 30 days. High quility filters are much better than changing more often than nessary regardless which household.