Hi I have 2014 plate 3.0d 5 series tourer. Over last couple of months the rear air suspension has played up. It has on a couple of occasions not raised the rear end up on starting the ignition. The wheels sit on the bumps and this is obviously a problem. The suspension then works if I stop the car and restart engine. It holds the height when lifted and rear end is even. Compressor doesn't seem to overrun once rear is lifted and I can't here any hissing noises from air bags leaking. Car has high mileage but issue has only started recently. Question - how should the system work? Should it keep the rear end permanently lifted and never drop down or does it do this in certain circumstances such tailgate open, fully loaded to ease access?? Any help suggestions appreciated. Thanks 

Sorry it's an issue with Rear Air suspension on a 5 series tourer 

It sounds like the compressor relay is sticking? That would be my first place to check. 
 

Plus 1 for Stu's diagnosis also check the ride height sensors 

The suspension is self levelling under load the ride height has a +/-  tolerance of 10mm From your description the suspension is either leaking or the control is draining the supply through the compressor (it has an exhaust) Once at ride height with no leaks the suspension compressor should be rarely heard, if there is a heavy load in the boot it should pump up to level the ride when the load is reduced the exhaust allows air out to maintain ride height/quality.

The airbag/spring with age can become porous where it rolls on its self in use check it carefully with soapy water it doesn't take much of a leak to let things sag. Changing them is a straight forward DIY job 

Good Luck 

Dave