Jeff's reply jogged my memory. I had something like this on my port CH270 (circa 1977 Chev 350 ci) engine. It had been squealing intermittently at idle for a while, and I thought it was the belt trying to drive the alternator when it was under load. Replacing the belt didn't help. I took off the belt that drives the alternator and the circulating pump and the noise stopped. Also tried it with the raw water pump belt off and the other on - the squeal was still there. I pulled the alternator and took it to an automotive electrical shop where they put it on their tester and spun it up -- no noise. It turned out the circulating pump bearing was squealing. The circulating pump had no outward indication that it was failing -- no drips, it pushed the coolant through the engine. Since this is the "normal" rotation engine, I put on a generic automotive water pump.
