buttanic

I have 2007 Suburban with the 5.3 flex fuel engine and 98,000 miles on it. For the last several thousand miles at idle there is a rattling noise coming from the front of the engine. I suspected a bad bearing in one of the belt idler pulleys. Using a mechanic's stethoscope I checked the all the idler pulleys, water pump, power steering pump, AC compressor and alternator. The spring loaded idler pulley on the serpentine belt seemed to be the source as it was the nosiest by far. I replaced it but the noise remains. It is the same with the AC clutch disengaged so it doesn't seem to be the compressor. Nothing seems to have rough bearings if spun by hand.