Hi guys,
thank you so much for taking the time to read and reply with your thoughts on my post.
I was away over Xmas and then when I next fired up the X5 I was greeted with this noise on a far more regular basis in a half-hour period, then a message on the dash "Level Control System Failure" with suspension icon, and after stopping and looking from the side I could clearly see that the back had dropped. After restarting the engine it did raise up again and get me home, but something is definitely on its way out!
Photos attached!
I contacted the guy I bought the car from and he told me the car came from auction and when he bought it the rear was on the ground and he replaced both air bags so it does point to the compressor (or a leak but that wouldn't explain the noise). Mudd46738 thank you for your comments and reassuring me on the DIY/ask a mechanical friend - I am going to see how tricky it is to get out and decide if to pickup a used compressor for GBP £200 or try a seal rebuild kit:
http://www.ebay.co.uk/itm/BMW-X5-E70...K-REPAIR-KIT-/
I guess I won't know exactly which part is to blame until it is on the ramp but might just buy both anyway so that I can fit the replacement compressor and then try to repair the old one in my own time...