broken sub or box flaw??

i built an mdf/fg sealed box for a single 12" xtant hex sub. when i really turn it up, it starts to make almost a tapping noise on the beats. i dont think its a bogging noise, and i tried almost everything to make a seal to see if it was a leak. i read in another post that if u hold the foam/rubber softly and the noise goes away, then that means the cone glue came off or something? ive tried re-placing the wires when i sit the sub in the box, since it sits pretty tight against it due to all the polyfill. the funny thing is when its at a medium or partly loud volume, it sounds perfect. once i turn it up, it makes that noise. and i know that sub can play louder, especially with the xtant class d amp, which was tuned correctly by my friend who knows his stuff. he said the noise was a leak, but now that a i read that guys post about gluing the cone back on, maybe thats the problem?