I have a 2003 GMC 2500 HD with Diesel engine. Has about 28000 miles on it. It has recently started making a surging-type or revving-type sound when idiling. Of course it doesn't ever do it for the dealer when I bring it in for service. The dealer said since there aren't any service codes coming up, and they don't hear the sound, there is nothing wrong. Anyone else experience this problem with their trucks? Any advise would be most helpful! Thanks!