smells,Moldy smell from AC at startup

I've seen lots of post on forums and websites about potential condensation collecting inside the ducting that is harboring microbial growth which is producing that smell.

Now my question is, why don't I smell it when I keep the AC off and ramp up the fan? This question may lead to the following: how do I find where the source is? Is the only way to thoroughly clean out this mess by taking the ducting/trunking out and clean/soak? Is it doable? I'm skeptical about those off-the-shelf sprays. I'm looking for a solution that doesn't just mask over the problem but getting