Land Rover Repair: Air Suspension Discovery II, car returns, whinning

Question
HI Dennis

I would like to ask you about the rear air suspension in my Discovery. Its a
2003 Series II with 70000 km on it. I have not had it long and love it but
would like to find out if what it is doing is "normal" or if its something I need
to sort out.

The first thing it does (and I think this is normal) is when someone gets into
the car and the engine is running, their weight immediatly lowers the vehicle,
then within a few seconds the car raises back to the normal/starting height. If
that person gets out, the car immediately rises but then within a few seconds
the valve hisses and the car returns to the original normal position.

The second thing it does is when I pull up at traffic lights, within a few
seconds, the value hisses and the back end lowers. This sometimes also
happens when parking.  So for example I might hear the value releasing 6
times in a trip from my place to down the shops and back. The local LR
mechanic (non-dealer) didn't seem that concerned and explained the valve
release occurs because when I brake coming into the lights the front end
dips, the back end raises. When I stop fully, it lowers itself back down. He
explains the hissing when parking as being due to parking on uneven ground.

The only thing that may be relevant is that I can hear an intermittent
whinning/buzzing sound (kinda like a fuel pump whine) I am not sure if its
related. Where is the SLS pump located on the car  ?

I am concerned that it is "hissing" a lot, virtually every time I stop, and the
whinning is the pump about to die.

What do you think ? Is this normal ? Should I have the mechanic hook
it up to the computer and see if there are any codes (there are no lights on
the dash) ?

BTW - It does not loose height overnight, the backend is always level and
both sides move together.

Many thanks for your help Dennis - You are providing a great service !

Cheers
Matt

Answer
The back not dropping, and raising immediately is a good sign.  

I"m not sure if the constant leveling is normal, I don't have much experience in a DII.  the compressor will whirr a bit, if it's whining there may be a problem.  The SLS compressor is under the drivers seat, bolted to the frame on rubber mounts.

If it happens more when you come to a quick stop, it's likely the mechanic's explanation is right, but if it happens even when slowly rolling to a stop there may be an issue with a height sensor.