HI:There is a condensate tray at the base of the evaporator(part in your furnace) that should be cleaned occasionally(bacterial growth) as it collects the condensate from the air(which is in turn drained out). The condensate tray should be installed a little angled toward the exit of the coil but over time etc this geometry can be negatively altered.
A strong bleach type cleaner is used  to deal with this issue.The evaporator coil can be (should be)cleaned which helps the over all heat transfer giving better performance and lower hydro bills.
Early in my career I was a manufactures service/application rep and coil clean up is often necessary especially if the problem occurs one summer to the next.