Air noise from ports is EXACTLY the reason I sealed mine up and bought a Passive radiator. Even at low volumes, the port noise was audible when the mains were turned off. To be sure it's the port, stuff a rolled up dishtowel in the port and listen again. If the noise is gone, you'll have your answer. If it's clipping, you'll still hear it.
Stryke.com (now acoustic Elegance) has an 18" passive that would suffice.
