2003 Ascender sings like canary



My 2003 V8 Isuzu Ascender (think rebadged Envoy or Trail Blazer, distributed by Isuzu)with 45,000 miles started chirping like a canary about two years ago (definetly not belt squeal or slipage). We took it to the dealer and he pulled the serpentine belt, cleaned and lubed all the pulleys and put on a new belt. This worked like a champ…for about a week. After another year of this the chirp had become much louder so I took it to my trusted mechanic. He cleaned the pulleys and replaced the serpentine belt. This helped for about three days. I’ve changed the belt tension and put a mechanic’s stethoscope on all the pulleys but can’t isolate the problem. There’s no noise up to about 800rpm. After that it’s pretty constant and doesn’t change pitch with revs. Short or replacing the air pump, water pump, air conditioner compressor and power steering pump, etc, any ideas?