disco 2 suspension

hi all . pick yur brains if i may. my suspension compressor will sometimes make a psssst noise, a little like the noise you get from an air brake, when you get out of the car. is this normal? if it isn't it and the compressor is goosed then might explain why the back of the car was on its bot this morning. i've read the difinitive guide to the disco air suspension and feel quite confident on doing whats needed but don't want to have to replace everything if i can get away with it. any help greatly received

Yes the noise is normal, it says so in the guide.

Sitting on its arse is not normal. Does it not rise when you start the engine and have all doors shut?

yeah it went back up straight away. supposed to be towing with it on thurs so don't know wether to just bite the bullet and order 2 new bags. car is comin up to 5 year old so could just need replacing as they recommend. only had the car since dec 08 and would think that they are original bags.