I noticed while doing an oil change on my 07 c15 acert yesterday I could hear a small air hissing sound from the oil fill cap with truck off and cap off. The compressor was replaced about a year ago because it was thought to be the cause of the air leak. It doesnt do it all the time, what could be causing this? Dryer check valve? Or is it normal? Truck runs good and builds air normal otherwise. Thanks for any help.