Smell only when using Cool setting?

Heil Dual Gas pack. Operates well. Heat is great and air has no noticeable odor.
However, when switched to cool cycle and air conditioning turns on... it smells as though it is pulling air from under the crawlspace or from a 100 year old house. (Home is 20 years old)
Switch back to heat and smell goes away. Duct work checks out fine - no leaks in return. Coils cleaned twice... no luck. I could throw a lot more money at this, but would rather get some advice.
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Will give an update as I diagnose and repair. Purchased a special bactericide / fungicide / sanitizer to spray on coils. They have been cleaned, but still smell. Next step, is to replace all insulation in the package and have it sealed. Then, replace duct work / replace the unit.
Frustrating. 5 professional HVAC companies - no answers.... yet.
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