I have the exact same sound at the moment except mine happens when the car warms up. What I noticed is where the belt loops around the idler pulley it "hangs" over the edge of the pulley by 0.5mm. I tried loosening the tension and pushing the belt back so it relaigned flush on the edge of the pulley. When I restarted the ca0,r the belt immediately shifted to the orginal "over the edge alignment" and made the exact same squeaking noise when it did. I assumed it was the idler pulley and ordered a pulley saver kit from Needswings. Even if that doesnt work, still cheap insurance. Just as an aside, my factory pulley did not have any shims and there is some every so slight uneveness on the pulley backing plate. Also since installing the new pulley I have placed approx 200-300 miles a week and have only encountered the issue only 2 weeks ago. I will subscirbe to the thread with great interest. Just my 2 cents.

Regards, Michael