could be way off on this but....
in the boot, front left corner, there is a flap you can lift up on the false floor. (where the jack is)
if you take the jack out and look right in the hole, there is a plastic flap (vent?).
when i installed my sub, it rattled like a mo' fo'. i could hear it outside and initially thought it was my number plate rattling. then i thought it was the boot opening mechanism. but a little more investigating led me to this.
solution: just stuffed some kitchen roll in it :P
btw,
'nearside'? that's the left(passengers) ay?