Sewage/Garbage Smell when upstairs AC kicks on

We live in a 2300 square foot, 2 story home in NE FL. (basically a main floor with a FROG) The house has two AC units; upstairs and downstairs that were replaced in 2019. (House was built in 2004; we purchased in 2020)

Over the years, we've noticed that when the upstairs AC kicks on, it smells like sewage/garbage. Just a yucky smell. We dont really notice it unless the air actually kicks on. The filters are changed regularly and that doesnt make much of a difference.

Any idea what this might be or what "professional" area this might fall under? (Plumbing, AC, etc)