Engine noise

My 87 GMC Suburban with 3436 miles on a Jasper rebuilt motor is making a screeching noise. It sounds like a belt is slipping. Whoever when I completely removed the three belts and ran the motor again, it was still making the noise. When the motor reaches operating temperature or when the motors rpm is increased above 3500 rpm?s the noise disappears. Could this be a internal noise such as a bearing or something else?