Does the sub do this only at high volumes. If so, you could be bottomin the coil out. If this is the case then you are just pushing it to hard. Either box size is too big or something else. If the speaker is thumping when you turn ON the stereo, could just be the amp warming up then giving it a signal( somewhat normal for some amps). But if it is random, take the sub out, check the coil, see if it smells burnt (very recognizeable), move the sub around outside of the box and see if you can hear any pieces of coil that could have broken off from bottoming out. If you still can't figure it out post more info and we'll see what we can do.
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