I'm so thankful for priuschat. Only recently I started hearing this sound on my 2009 and it really got me worried. I'm happy to hear that its nothing functionally wrong with the car nor potentially dangerous. I'm still under my 3yr/36k (currently at 27k), so I'd definitely going to have it fixed. Is there an updated TSB covering 2009 models? What is a little annoying is that it sounds like many of you need to do a bit of coaxing to get the dealer to work on what is obviously a defect that should be covered by the warantee. I hope my dealings with the dealer go smoothly. Wish me luck. Just to add, it happened just this week when temperatures dipped into the 20Fs, so I'm inclined to believe its cold weather related.