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.

Sounds like a new belt was slipping and glazed a bit. Remove the belts and see if noise goes away.

Thanks Dennis. So I assume if the noise stops, put them back on one at a time. When the noise returns, how do I determine if it is the belt or something like alternator itself? Not familiar with the term glazed belt, what's the fix?

Glazed belt means the belt was slipping a bit and the sides ended up polished. Once polished, belts will have a tendency to squeak especially under quick engine acceleration. Generally happens when a new belt stretches a little and gets loose.

Give accessories a spin by hand before putting the belts back on. Likely can feel a bad bearing if not the belts.