sienna whistle noise!

Hi every one! I got 2001 sienna and had 90k km on it, the engine look ok no oil leak or anything but when I start the vehicle in diffrent weather condition, some time I heard a sound like whistle noise came out of the alternator. The sound came off after warmimg up or it will be nothing in another weather condition... I gessing the bearing of the alternator is getting dry or turning bad, so anyhow can I grease it or lubricate it? How are every one think about this? is that the problem or something else. Thanks in advance!Sunsettt

could be the bearing. my car has been doing that for about  a year. Unfortunately they are a sealed bearing and the only way to lube it, is to replace the bearing.