I have a 08 3.0 model. So no rear air suspension.

When going over a steep speed bumps or an obstacle, I hear a short hissing noise like 'whoosh~' coming from the suspension; sounds as if air being released.

I understand the shock absorbers are gas charged in the X5, but is it normal to hear that noise from inside the cabin? The gas is supposed to be pressurized, so it shouldn't escape.
Do any of you have this problem?
I thought you only hear that in old cars with worn out dampers.