subs making pop sound

hello i made a ported box about a year ago but never put subs in it ...so i got another car (91 caprice)an put 3 alphasonic 15's 100 series...the box is 9 cubic feet and i can not remeber what the port size is..but i get a almost like a pop sound out of all the subs ( they are new)..could it be that they are under powered i just hooked up a clarion 75x2 to see if the box was going to sound ok before i move any futher.....or could it be the box? any info is welcomedA way of life..

If I had to guess, you are probably bottoming out the subs.  If that enclosure is tuned really high, and you are playing frequencies well below tuning, you could easily cause this.  I'd suggest modeling those subs up with that enclosure and see what the response is.  Also check the settings on the amplifier to be sure they are set correctly.