Thanks flapjack, but that dosn't seems to be the problem.....?

Regarding to the answer by flapjack to In the first place I don't turn it up loud, I've a 559 amp by yamaha, and normally I would turn the volume to the most -18db, LFE level -2, crossover of amp and sub to be at 80hz, volume at sub at half. Anyway the yamaha techinician also thought it's overdrive in the 1st place. But after replacing the new driver from japan, the sound is still there, and after flidding around with the controls and everything, it's still there. Then they bring back my sub and change everything inside except the wooden block, the sound is still there. They bring in a new set from their supplier, and the new set also have the "bubbling" sound. Seems like this particular model have this problem, that's why now they've reported this issue to Japan side, I wonder what about the other model from Yamaha now...