s320 airmatic height

hi i have a 2000 s320 with airmatic and have noticed (also people have commented) that it sit's low at the front, when sitting normal the height from the top of the wheel to the arch is just under 1 inch but the back is about 2 1/2 inches, when i lift it up the car will go up but there is still 2 inch difference also sometimes when its stood i can hear a slight hissing noise from the front as when i exit the car the front will drop slightly, so does anyone know the correct height it should be (my neighbours friend has a 2006 s320 cdi and his sits level and higher than mine?
cheers
mark