Door speaker knocking sound

[sandyman65535]

My stereo plays quite well, but when I play a particular song (Blowout by The Crystal Method - can listen here if you wish, accompanied by some average mountain biking) there is a knocking sound from the driver's door speaker about every 8 beats when the bass drum hits. It sounds like if you knock a knuckle on a wooden desk and stands out alot from the sound. I know listening on different systems, such as headphones, gives a different sound (and is part of the fun), but this is just weird.

That's the only song I've heard it in, so I don't play that song any more because I don't want to think that my beautiful car is broken.

Can anyone tell me if that indicates that the woofer is shagged? And is there a definitive way to test it?

Thanks,
Sandy.