Was the smell strong? Might be just a small exhaust leak or something. I kind of notice a smell like you are talking about when my truck is under load. I thought it was maybe the EGR or something, but you don't have EGR on your 04. The smell is very weak and comes and goes like you are talking about. Not too sure what it is.
2002 Silverado 2500HD LB7 CCSB LT
EFI Live by Rob at ATP, Edge CTS Insight DSP 5 control, S&B CAI, PPE boost valve, Pro Fab AIH, AirDog II 165, PPE WIF sensor delete plug, 5" stainless exhaust, Cognito braces & HD tie rods, H2's 315/70 R17 BFG's
2004 Tahoe LT
2006 Silverado Z71 LS
1971 Chevy C10