The port is chuffing

Your port's air velocity exceeds the threshold at the level simulated. Past roughly 17 m/s, flow through the port stops being smooth and turbulence becomes audible: a chuff or a whoosh on bass notes, uncorrelated with the music and quite distinct from distortion.

That 17 m/s figure is guidance rather than physics. It depends on the port's end geometry, its aspect ratio and the listener's distance, and published thresholds range from about 5% to 10% of the speed of sound. A sharp-edged port misbehaves earlier than a well-flared one.

What to change

1. Flare the ends. The cheapest real improvement. A radiused or trumpeted port end delays the onset of turbulence substantially, and a flared port can often be run well past the velocity a straight one tolerates.
2. Increase port area. Velocity falls with area for the same volume velocity. The catch is that a larger port must be longer to keep the same tuning, and length grows fast: you may run out of box.
3. Use more ports. Two smaller ports have more combined area than one, and each is shorter than a single port of the same total area would be.
4. Use a slot port. Easier to make large in area, and it can run along a box wall where a round port would not fit.
5. Raise the tuning, which reduces the excursion the port has to accommodate, at the cost of extension.

When to ignore it

Velocity peaks are narrow and level-dependent. If the flag appears only at a drive level far above what you will use, it may not matter. Check the drive voltage the simulation is using before redesigning anything.

If the port cannot be made to fit at an acceptable velocity, that is real information: the design wants either a bigger box or a sealed alignment.