95 C1500 has extremely high pitched squeal coming from engine area

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Have replaced the A/C compressor, power steering pump, harmonic balancer, fan clutch plate, tensioner pulley, idler pulley, alternator and the distributor. All of these parts were not changed due to high pitched squeal. Upon further testing they were found to be faulty and/or going bad. Truck has 269,000 miles and engine runs very strong. There are no misfires. Other than high pitched squealing/squeaking at LOW RPM's truck has absolutely no issues! What could be causing my annoying problem that is driving my wife insane? Now, after replacing all of those components, theory would suggest a severe leak in the intake manifold or TBI. Have completed preliminary tests with starting fluid sprayed around TBI and intake manifold, NO change in revolutions or high pitched squealing noise! I am open to ANY good suggestions that make sense and will be able to correctly diagnose and correct the issue, OTHER than a complete engine replacement or overhaul.