air suspension inactive?

[gem03]

Hiya

This is my first post, so hope this is in the right place!
I have an X reg BMW X5 4.4 V8 sport and after driving it a few days ago there is a message on the dash saying ' air suspension inactive' at the time the car wasnt any different to drive. However over the following days there was a hissing noise coming from the back wheel area and also a noise from under the spare tyre in the boot, like a pump or motor which didnt make a noise before.
These noises are mainly as soon as the ignition is switched off after driving. It then got worse with the car looking lopsided from the back, one side lower than the other.
And now both sides are down, it cant be driven as its like driving a tractor! No suspension whatsoever, so if you hit a bump in the road etc you know about it.

But anyway took it to a local garage who specialises in high performance cars, 4 x 4's etc and was told my car has been on the diagnostics and they cant find anything wrong with it.
So it hasnt been driven since then ( which was 3 days ago ), my husband took a look today after reading some things online and found that one of the fuses is blown ( 87 i think ) and if its replaced the cars suspension does go back up, but then goes down again slowly with the hissing noise and the noise in the boot ( pump/motor ) still there once the ignition has been turned off. So it doesnt fix the problem just by replacing the fuse.

Does anyone have any idea what this problem could be? As we are at our wits end and dont really want to take it to the main dealers for obvious reasons.

Thanks alot
Gem