Bose Subwoofer

No it's a common problem. Either the woofers are blown, or more likely the middle screw holding the front panel to the tupperware enclosure might have cracked the housing. Try pushing on the front of the enclosure (where the speakers are) while playing and see if the distortion disappears. If it does then it's the screw problem. You need to remove the sub, and araldite the broken part then carefully tighten it back up but not overtighten.

I don't have an A3, so I'm not sure if there's space (hopefully AndyMac can confirm), but it might be worth getting one of those squidgy vibration damping washers that you get with computer noise reduction kits: they give you a good way to make sure something is very secure without over-straining whatever it is you are screwing through (and is a good way of stopping things from rattling/distorting).

I can't find the washer-set that quietpc.co.uk used to sell, but a bit like the ones in this CPU cooler kit: (LINK)

edit: here we go: it was the Sharkoon Silencer Kit (LINK) or £8 from Pixmania (LINK) - obviously a bit much if you only need the one washer though.