2007 Silverado NNBS LTZ VMAX 6.0 4X4: Happens after warm up. Goes away for few minutes if I shut off and restart. Doesn't change with engine revs, gear, 4wd or 2wd. Don't notice any change in way truck drives. Sounds like maybe a solenoid or relief valve opening and closing? Listen to link. Any idea what it is?