On my 2003 there was a "filter" on the drivers side, maybe you are hearing air trying to enter it ? Really wouldn't think you would when the truck/compressor is not running though ?
It was rusted pretty bad causing poor air intake to the compressor making it work harder and it then cause the compressor to shut down till the next time I started the truck.
There is a picture of it here
http://www.elcovaforums.com/forums/s...ad.php?t=24471 scroll down to post #12 to view it.
Ed