Some early 1986 'G' models biilt through 11/15/85 (through Chev VIN 121678 and GMC VIN 507587) at the Scarborough Assembly Plant (VIN Code 4) with 6.2L diesel engines may have insufficient crankshaft pulley mounting bolt torque. A noise coming from the front of the engine could indicate this condition.
With the engine off, inspect the pulley, belt and fan for visual damage. Then check the torque of the four pulley mounting bolts. If torque is found below 34 N.m (25 ft. lbs.) remove bolts and pulley and inspect for damage. Replace any parts found to be damaged. Upon installation bolts should be torgued to 34-48 N.m (25-35 lbs.).
General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.