Why is the Jeep squealing constantly?

My boyfriend bought a used 93 Jeep Cherokee Sport 2 wheel drive 6 cylinder engine last month and since he had it the Jeep has a high pitched squeal to it when he accelerates and while in Idle. He bought some belt dressing and at first it made the squeal stop while the Jeep was ideling and it got louder during acceleration but soon wore off and now the jeep is constantly squealing. He replaced the Idler Pulley in hopes that it would work but it just made the noise worse. Does anyone know how to stop this horid noise?