Ok so I made a few boxes for my hdc4 and it sounded good except on a particular note. I believe it was a higher frequency seeing as how I still hit the lows and moved air fine. If you listen to "candy shop by 50 cent. It's that bass note through out the song. My sub makes noises like a knocking noise. I can hear in the port and I can hear it when I listen to the sub. Any ways I thought it was the tuning so I made quite a few boxes but had same results or less noticeable, but still there results. I'd listen to it free air and it sounded fine. I even switched the subs to a different brand and it didn't make that noise. So I was assuming it was the speaker. But then I tried it with the candy shop song because when I tried it free air I kept forgetting what songs it did it on and sure enough, I had the sub on my arm rest/center console, and it made the noise! I was like wtf. But as I tilted the sub it went away slightly, so I layed it down, so there was no obstruction to the air coming out the motor. And it was fine! I put my hand over the hole and it made the noise. So I'm thinking my sub is mounted too close to the back wall. My box is 15" deep gross, and I have my sub forward firing, so the distance between the wall and sub is like 5.75" I emailed soundqubed and they said needs to be a minimum of 6" away, so I'm gonna double baffle it witch will put me at 6.5" away. Do you think that will make a difference? Is this the kind of effect being too close to a wall have? Now that I think about it, I haven't really build a box this skinny lol. Maybe the same height but depth would he like 2 ft. Lol. Hope to hear some inputs from you guys.