'95 Century AC mildew?

Turning the AC on generates a mildew smell that seems to go away after about a minute.
My question is: what most likely cause this smell and how to fix it?

To start make sure the HVAC condensation/water drain is clear. The mildew smell is normally from mold or fungus. You can get special aerosol spray from auto parts stores and run it through the system via the cowling air intake to minimize the smell. Also, limit use of A/C MAX or Recirculate settings and use Normal.

Thanks , BNaylor for the reply.
Do you know the exact location of this drain tube?
I looked for the drain tube & was unable to locate it.