Have found out anything yet? If so, or when please post the findings and results. DON’T leave us Hanging!:shrug:

We have been so pleased with M&S air conditioning company. They have done inspections and large projects for us and everything has gone well. Good luck!

Sorry for the delay in getting back to everyone who was kind enough to give suggestions. I decided to use ServePro to inspect and service the ductwork. They create negative air pressure in the ducts and suck everything out to a big unit in the garage. Supposedly, ducts were then cleaned by a flexible air whip type device which doesn't harm the ductwork. I could not watch the process in the attic, old man here! I'm assuming everything was done as stated! The Tech said nothing found in the ducts but some original construction very light dust, etc. No moisture, dead animals, etc. Then everything was sprayed with a great smelling product through the air handler. All was fine for about 48 hours, then the odors returned! SOB!! OK, I opened the air handler and reinspected the coils, fins, tray, etc. I sprayed commercial grade Lysol wearing gloves, mask, and my shooting glasses on the tray and both sides of the lower section of the coils, etc. Carefully wiped down the tray and found some black mold under the tray and in the tray near the drain. Sprayed that area heavily. Also saw dark streaks along the bottom area of the fins on the top side that I must have missed previously. Sprayed the bejesus there as well as I could reach. I also felt as much of the return air chamber as I could reach. No visible mold came up on my Lysol-dampened cloth. Gave a light spray to all sides of the return air chamber. We split and left the house for a few hours. It's going on 10 hours and everything smells half decent so far. Tonight will tell the tale! BTW: Great difficulty finding any AC company who actually want to touch the air handler section or do any hard work whatsoever! I don't trust any service people, got burned by several in the last 4 years here. OK, rant over. Another report may be coming.
Thanks, everyone, some great people on here!

Thank you for follow up.

OP, I hope you had a discussion with ServePro for at a minimum a discount on the service call.

They should have cleaned the coils and pan as part of the service.

Have them come back and finish the job now that you identified the source.

Our HVAC guys put a few of something like this in the pan at maintenance.

https://a.co/d/e7MpcI3

Do you flush the drain lines monthly?

Builder recommends hot water with a few drops of Dawn.

Depending on the air filter, some are not UV light resistant. You may need a UV resistant model of the filter.