Not sure if all cars have these as standard, but my car has a cable hanging down behind/inside of the rear bumper which is used to connect the lights upto a trailor so that when your towing a trailor the lights on the back of it work at the same time as the ones on your car. This used to rattle and bang around when i drove over bumps in the road until i taped it down to the inside of the bumper.

Maybe this could be what is causing your problem?

YEAH IT'S LIKE SOMETHING UNDERNEATH AT THE BACK IS HANGING OFF.. BUT WITH THE WIND IT'S SO ANOYING!! SOMETIMES I GET A RATTLE AND IT'S SOUNDS LIKE THE ZORST BUT HAD IT CHECK OUT AND NO PROBS!! STRANGE... NOT THE WATER PUMP COZ GOT A NEW 1 AND NOISES COMING FROM BACK!!

Is it like a rumbling noise?

It's either a) your aerial vibrating as wind passes over it...
or b) wind pushing against the rubber door seal...

This is assuming your engines not running, in which case it could be something to do with that...

I get a very annoying noise too when its windy. Ive had it for about 2 years now, but i can't find where its coming from.

Could be the zorst, same sorta noise on mine. Had all the fittings checked but its all good so no worries!! Oh well

i had it. if its comin from left hand of engine. its the water pump. needs replacing. garage will charge bout £120

They are so anoying though arnt they ive got one but can i find what it is!- just have to live with it til something falls off! LoL.

I get that when its stood still, sounds like a birds stuck under the car or summin. Never found out what it is but its not given me any problems.

REAR PASSANGER SIDE HAS A REALLY ANOYING RATTLE WHEN IT' WINDY TOWARDS BACK END.. ANYONE ELSE HAD A PROBLEM AND KNOW HOW TO SOLVE IT?? CHEERS