Moldy mildewy Smell when central AC is turned on after a few minutes

[Pepper Lab]

House is 31 years old ranch
Trane Furnace is 3 yrs old
Central AC Lennox Elite series EL16XCI is 15 months old
APRILAIRE 5000 Air Cleaner 3 yrs old
Aprilaire 800 steam humidifier 3 yrs old

A few weeks ago it smelled wet when the AC came on. Now the smell is a sick strong mold mildew smell.

We always had to use a dehumidifier in the basement during spring when water enters the basement due to cracks in basement foundation. But when AC ran during summer the basement remained dry in past and it rarely turned on. It is now running at same time and just as much as AC.

Never had this mold smell problem previous, the coils from original 16 yr old AC system were just like new no mold or contamination when removed for the new system install 15 months ago. Nothing else in house has changed since AC was installed.

I am being told I may consider a whole house dehumidifier mounted to ductwork on furnace, UV lights, return air ducts installed in unfinished basement with no wall partitions.
This is a new problem that has NEVER occurred in previous 31 years.

Furthermore, we heat house with wood for past 9 years and 3 years ago we had installed an Aprilaire steam humidifier. The steam outlet is mounted centrally above the coils as they were on old coils.

How would a UV light work with that steam tube so close by. I have read various opinions that UV light could help inhibit future mold growth.

WE have to remove a shelf unit in order for a bypass dehumidifier unit to be installed. More expense I know dollar figures are not mentioned here but that is what I am seriously lacking.

I am unable to do much of anything due to wheelchair use for past 6 years and never expected all these problems with a new HVAC system.

Any suggestions on light, dehumidifier, or what may be happening, but without seeing problem... IT STINKS

[teddy bear]

IN an effort to dryout the ductss, operate the a/c blower for 10-12 hours everyday. Try this for a week. Monitor the %RH, looking for <50%RH in the home. Also supply/space temp/%RH to determine the a/cs ability to remove moisture.
Any constant wet spot in the equipment can cause this. Drain pans must drain completely. Also drains without water traps may allow air from the trap into the a/c.
I assume humidifier is inactive for sure.
Keep us posted.
Regards Teddy Bear
Installing a whole house dehumidifier with the dehu supply into the a/c supply ducts would extra dry the a/c system. The install connections will be critical. Also enough capacity to throughly dry everything is important. If indoor air quality is critical, consider fresh air inlet to purge indoor pollutants during calm summer weather. Also check basement drains for dry traps that would allow surr gas to get sucked in to the home. Also rotted vent pipes leaking when the a/c blower runs.
Check out the Ultra-Aire whole house dehus for the most efficient and best ventilation control.