IT'S LIKELY THE OIL INTER-COOLER / EGR MANIFOLD ASSEMBLY BOLTED ON THE SIDE OF THE BLOCK UNDERNEATH THE INTAKE MANIFOLD. I HAVE FACTORY SHOP MANUAL & IT DOES NOT ACKNOWLEDGE THE EXISTENCE OF THIS SIGNIFICANT PART - NOT EVEN IN THE ENG. TEAR DOWN SECTIONS. THE MOST LIKELY PART OF THE ASSEMBLY THAT IS LEAKING IS WHERE THE INTERNAL OIL RADIATOR LIKE SUB ASSEMBLY BOLTS TO THE HOUSING - THERE ARE 2 THERMOSTAT SIZED & SHAPED COPPER GASKETS THAT WILL LIKELY BE THE CULPRITS. SUGGEST PRESSURE TESTING THE INTER-COOLER ASSEMBLED & BROKEN DOWN (OIL RADIATOR REMOVED & ISOLATED). THE OIL RADIATOR HAS STUDS THAT PROTRUDE OUTWARDS TO EXTERIOR OF THE HOUSING IN WHICH YOU CAN SEE THE 14 MM NUTS- 2 TO THE LEFT OF CENTER & 2 TO THE RIGHT OF CENTER - THAT RETAIN IT. THE REST OF THE BOLTS ON THE EXTERIOR OF THE INTER-COOLER ARE THE HOUSING'S MOUNTING HARDWARE. RECOMMEND TO LEAVE 14MM NUTS INTACT UNTIL REMOVAL OF HOUSING SO AS TO BE ABLE TO INSPECT COPPER GASKET MATING AREA B4 DIS-ASSEMBLY. CHEV DEALER PARTS DEPT. DOES HAVE SCHEMATICS OF THIS ASSEMBLY & HAPPILY PRINTED THEM OUT 4 ME. ASSEMBLY COSTS $691.00 - BUT IT IS LIKELY YOU WILL ONLY NEED GASKETS & O-RINGS TO SOLVE PROBLEM. HAVE PICS OF ASSEMBLY REMOVAL STEPS ( DID NOT HAVE TO REMOVE FUEL PUMP) & BROKEN DOWN INTER-COOLER ASSEMBLY IF NECC. I'LL CHECK THIS POST EVERY FEW DAYS TO SEE IF YOU WANT ANY FURTHER INFO. 2004 CHEV T7500