Hi guys. Recently my Mordaunt Short MSW20 sub has started to produce some strange noises. Certain levels of bass seem to cause it to make a rapid rumbling noise more akin to someone blowing a raspberry than proper rumbling a sub should make. I do wonder whether it is nearing the end of its life but am reluctant to switch to a new sub at this stage due to finances. I have checked that all the levels are right on my amp (Denon AVR2310) and that the polarity on the sub is set to 0 and the frequency is set to max (crossover is set to 100Hz on the Denon) and its "volume" is set to 50% as per the recommendations on BatPig's setup guide for the 2310. Any ideas on what might be causing the sub's odd noises would be greatly welcomed.