air bags and compressor question???

howdy all, I was wondering if anybody can tell me if this is normal or not, I've got a disco 2 v8 with air bags on the back, lately if I start it up and do a short journey or move it off my drive on to the road side, when I turn the key off and get out for a few seconds after you can hear the compressor kicking in, in short bursts, is this normal or could it be a leaky bag or problem rearing its ugly head with the compressor?????

any help greatly appreciated, and I must say its not all the while, and strangley I've just moved house, my last house had a tarmac drive and was perfectly level, where as my new house has a rough tarmac drive but is not level and has a slight slope across it could just be coincidence??????

cheers andy