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I think that you may be on the right track based on what I found recently on an engine that I was working on for a friend.
The engine was a relatively fresh rebuild with probably less than 1000 miles on it.

The engine would develop a high pitched chirp only after it warmed up for about 3-4 minutes. After that the chirp was there no matter what until the engine was shut off and allowed to completely cool down.
The cycle would repeat again once the engine ran for a few minutes.
The chirp was more pronounced in general on the passenger side of the engine.

First thing I did was to remove the fan belt but the chirp remained.
Swapped the distributor with another. No change.
Listening with a mechanics stethoscope I could hear the chirp very clearly on the #1 spark plug.
I switched the #1 and #4 spark plugs. No change.
I could also hear the chirp more clearly when pressing the mechanics stethoscope on the intake manifold. The chirp was louder on the side closest to the #1 cylinder.
I finally ended up removing the valve cover and using a pliers I turned each of the valve springs one half turn along with the distributor drive gear spring. Buttoned it back up and the chirp was gone.
I suspect that it was the spring on the #1 exhaust valve that was the culprit. I was tempted to only turn that valve spring and leave the others alone but I figured it would be prudent to turn them all when I was in there so it is possible it was one of the other springs but I sort of doubt it.
I must admit this one had me loosing a bit of sleep before I figured it out.