Creak/Squeak from rear suspension

[JBF]

I'm hoping someone can help me. Here's a link to an audio file:

LINKY

This soundbyte is in three parts. The first part is me driving along in a very flat multistory carpark with my iphone hanging out of the window (you can hear the wind). The second bit is with the phone sitting on top of the drivers side rear tyre (RHD car) while I stood on the running board, held the roof rail and rocked the car. The last part is with the iphone sitting on the passenger side rear tyre with me rocking the car in the same way.

This sound started as a slight creak/squeak when I went over bumps slowly, like the small stubby rubber speed bumps you get in car parks. When it was in the dealers for a service I asked them to track it down but they claimed not to be able to reproduce (yes they are totally useless). Over the last month or two it's gotten much worse so that now it creaks constantly with every little bump/lump in the road. It can't be heard inside the car with the windows up but is very easy to hear with a window open slightly - this is becoming quite embarrassing as it's quite loud and people look when I drive by!

It's definitely much louder on the passenger side. Sounds like whatever it is resonates through the wheel/disc although I very much doubt they are the source. I suspect the air spring, shock or a bush but hope with the number of X5 owners on this site - somebody will recognise it and be able to point me at the right component. I can't imagine this is uncommon as I'm only at 64k miles and the car is well looked after.