During the summer the drivers door main speaker started to sound like it was damaged. It got worse with the Bass settings at the max.

Had a quick look around, the speaker seemed intact. Local dealer looked changed the speaker during a service, the problem was still there. Swapped the speakers over (from door to door), the distorted base sound stayed with the drivers door.

Went to see the dealer who partically moved the speaker away from its mountings in the door, the distortion seemed to be less pronounced.

We used a screwdriver to hold still the internal waterproof doorskin (grey foam).

This seemed to be the problem, I'm guessing that the hot summer we had must have loosened the doorskin so that it vibrates.

Dealer has ordered a new skin.

Will let you know how it goes.

I've read all i can find on here on this subject, mine makes nasty "farting" noises too - I've come to the conclusion it is the foam membrane pumping as the speaker cone moves because there is nowhere for the air to go, there is effectively a small volume of air trapped between the back of the speaker and the memrane

One of my front speakers distorts too, it go's away if I open the window though? must be something to do with pressure.