I have a 68 400 with a THM 400 and power steering. Recently I started to get a screeching noise consistently at 3500 RPM. Happen in gear and in park. I need some advice on how to troubleshoot if it is a belt, alternator or PS bearings, tranny or something else? Belts are in excellent condition, changed a few months ago. The noise did not coincide with the belt change - about a month gap before the noise started.