Problem diagnosed.

The noise is most certainly the air compressor for the airbag rear suspension. Three days ago after having driven the car for the day I went to the garage and noticed that the RHS had drooped significantly. Upon inspection the LHS had also drooped, but not as much as the right. I started the car and then the noise that I thought was the air compressor started and the bags both inflated. So now I know for sure and for certain that the noise is the compressor.

I have ordered 2 brand new air bags and will install them when they arrive. Considering that the X5 now has 150,000km on it and has not had the airbags replaced I expect that they willed replacing.

I say expect they will need replacing because it could be a fault with the pump that sees air leak from them. Air bags are cheap so I do not remind replacing them preventatively.

While I am replacing the air bags I will inspect the pump for any obvious issues. If none present themselves and the noise continues post bag replacement, then I will investigate replacing the pump. The sumo too can be purchased quite reasonably.