My suspension, is this normal?

[michaeljr6]

My 2007 e70 rear suspension would sink down after a couple of hours parked. However right when I press the unlock button and open my door, the suspension would go back to normal and makes a buzzing noise. Once it levels itself and the buzzing sounds stop it looks normal again. Should I go to the dealership and tell them about this? Or is this normal. It only does this when it's parked, the only thing that sucks is when I'm in a hurry I have to wait for the suspension to raise up or else I will bounce everywhere on the road. So what do you guys think happened?