2001 BMW X5 haveing problems with my back air suspension on

hello , my name is fridys . 1 week ago i was going to work .wen i wentt drive my 2001 x5, i saw the back of my x5 lower then the front. so i open the door to drive, and heard a noise of kind of like a small moter hamiing noise, fo a few secounds. when i
turn on the x5 the noise stop.also it went back to normal ,back up from the back. what can you tell me
about this problem or what part should i buy. please help me. thank you fridys leyba
my phone # 305-631-0013