I also have this problem with a 2004 Toyota Sienna. It seems that when it is cool and/or damp, then I get a whistle or squeal from the serpentine belt. I have not been able to get anyone to confirm how to adjust the belt and have not yet found the Haynes or Chiltons manual for this newer model. I'm not sure if this is the proper procedure but I WAS ABLE TO TIGHTEN the belt a little. There are two bolts on the alternator and an adjuster bolt. The ones facing the passenger side at the top rear and bottom front hold the alternator in place. By loosening those, I could then turn the adjuster bolt at the front bottom of the alternator. Due to the angle of the bolt, I needed two long extensions and a swivel. It's quite a trick using the swivel and keeping the socket on the bolt. I will watch for a while and see if I continue to get the squeal. I found one other query posted on the web about this squeal problem and mention that the dealer was not able to fix it saying it was normal. More to come if I get any new info.
Thanks for any input anyone can give.
This worked fine for me on our 2000 Sienna. I just would add the adjuster bolt did tighten
in the usual direction -clockwise. 1/4 to 1/2 turn did it. I would say just turn enough to stop the squealing.