w168 V220 air suspension

I just bough a v220 with airbag suspension and I'm concerned there is a leak. The rear falls to the ground in a day. Is it supposed to do that? I haven't heard the compressor work while I'm driving but every time I start the car it has to pump to lift the back. The front doesn't deflate at all. I'm assuming there is a leak. How do I make sure and how much would you estemate the cost to repair it must be? Also there is some whining noise from the engine when its under load. Its not loud but noticable. Any ideas?