Yal has it right. It's the fan man!!! Well at least you know that's working OK. Alternator bearings are permanently lubed. if they were bad, you'd be hearing their song all the time. Fan does not "click" in and out, but rather engages quietly as the temp of the air coming thru the radiator increases. Your description of the temp situation really points to viscous clutch and fan as making the noise. This is normal. when things cool down a bit, the clutch releases, ergo, the fan becomes quiet.
[This message has been edited by jeffsr (edited 06-19-2000).]