More than likely its the downwind leg of the ductwork leading from the
Some mold probably formed there, and now getting a fresh dose of humid
air has caused it to proliferate.
If your humidifier is easily slipped off so you can get to this area
of the ductwork, spraying some bleach inside the run will kill the
mold, follow up with a rinse of clear water. Give it a little time to
air out and dry before turning everything back on.
FWIW: Most of the nation has come down with flu like colds, it passed
through this area last month and had the whole household down twice
because of it.