Envoy noice in the back of 02 SLT

When my car is in park position or still position then I hear noice from the exhaust pipe somewhere in the back and it stays for 10-15 seconds and than it sounds like that some air releases and that noice stops. Normally it starts when car has been running for a while. This sound is like some growling sound,

This is probably your rear auto leveling suspension working.

Is this normal to hear such sound. Even with no load in the car. What does this system exactly do ? Pls. help me if you can. I do hate this sound each and every time driving. I have the feeling driving some large trailer truck...

it's your air compressor recharging, it loses air for no known reason and there isn't any way of turning it off, sorry.

Big Buck.

Hi, I have a question. A couple weeks ago I heard a strange creaking noise coming from the back right of my Envoy. I got out and pushed the car and the noise intensified. It continued creaking the whole day while I was driving, but that night the noise almost disappeared. I can still hear it, but I have to really listen. Also, I noticed that I hear it most when it's moisture outside...I hear it much more when it's raining outside. (I know it may sound weird, but that is the case)

Any similar experiences or solutions?

Thanks