How to Fix Central AC Bringing in Outside Air - Smokey outside
There are two ways I could think of that you could be drawing in outside air:

The return ducting runs through an unconditioned space, like an attic or crawl space, and has a leak, or
The supply duct runs through an unconditioned space and has a leak.

The AC system should be sucking return air, heating or cooling it, and then supplying that same volume (approximately) back to the house. In the first case above, the AC intake is sucking from the return duct but also from the outside, because the return duct is at a lower pressure than the outside.
In the second case, a hole in a supply duct means the conditioned air is shooting out of a high(er) pressure duct to the outside. You might not think this is such a big deal, but that volume of air being lost means the AC intake is now lowering the pressure of the entire house trying to find makeup air to keep the total air volume the same. This means air will be sucked through any crack in your house to try to equalize the pressure - door seals, dryer vents, window sills, bathroom fans, etc.
Sometimes a window leaks bad and you can see the curtains rustling, especially when the wind blows. If the leaks are bad enough, the air will permeate through the house regardless of what the AC is doing.
Since you're saying the smells only happen while the AC is on, I would guess it's a leaking duct, again in an unconditioned space. It's not immediately clear to me how your return is setup, but I would guess there's a large intake grill on the other side of one of those walls that just dumps into the air handler intake.
That would narrow it down to an air leak in a supply duct in an unconditioned space. I would check for it by starting in the attic or crawl space and trying to listen for it. You might also be able to feel a temperature difference in the insulation.
Finally, the spots for leaks would probably be at the joints where sections meet, like elbows. If I had to bet, I guess something like a sagging duct under a crawlspace ripped open an elbow flange and the air is shooting out.