Smell in 99

I recently acquired a 99 Astro and am experiencing a smell. It smells like oily mildew and is a little more noticeable when I first turn on the heater. If there is mildew or bacteria in the ducts how do I clean it out? Or what else would create that conditon? Someone suggested spraying Lysol in the air intake though that sounds like a bandaid approach to me.

Thanks,
Sixty