On my 2002 2500 Avalanche when I restart it hot, there is a growling howling shudder vibration for about two seconds after it starts. It runs perfectly well, but when there is a passenger that hasn't heard the noise before they jump when they hear it. The dealer couldn't find out what is doing it, as the noise is gone so quickly it seems impossible to pinpoint. Has anybody else noitced anything like this? It is like a "BrrRoar".