i have a brahma in a 1.8 cu ft box tuned to 30hz...the port is 4" PVC with a 90degree cupling. (port makes an L-shape)
when the sub gets loud, i can hear what i think is port noise...it doesnt sound like it comes from the cone of the sub...its like it comes from the port
the only way that i can think of is build a larger area port, with a shorter port length. i was told that the smaller the area of the port, then the more noise it will make. i don't know how to calculate this, but you can use one of them online calculators. sorry, i don't know the links to it right now.
To get the same tuning, a larger port would actually have to be longer. Try filing the inside edges of the port to begin with. If that doesn't work, you'll need a larger port.
Its the 90 degree coupleing. If it couples inside the port, i is distracting the air flow. You need to get a coupling that couples out side the PVC, bigger. That way there is no air flow disruption. Because really a 90 deggree is not part of the port (tuning). Its more of a re-director.