Noisy Drive Train

I've got a Shimano 105 equipped bike (Specialized Allez Comp Triple) that I've been riding for two years now. I just got it out for my first ride of the year and I noticed a "bird whistle" like noise coming from the chain and sprockets. I've associated this with a dirty, greasy drive train in the past. But, that's not the case this time. I've got a new chain on, and the chainrings and cassette are as clean as I've ever had them. Does anyone have any ideas what this noise is?

 

This is probably a dry jockey/idler wheel bearing - 105 is notorious for this. Strip down both bearings and regrease them. Note that the wheels and their bearing components are not interchangeable.

 

Ah... Excellent--thank you!

 

Ah... excellent. Thank you!