I know this is an old thread but want to contribute for anyone with this issue... The defrost vents on the dash just pull out by hand, I didn't even need a pry tool. There are no screws or clips, no wonder it is loose. The circle thing is a sensor, probably a light sensor so your automatic headlights know what to do. Just push down on the dash and jam your finger under the vent, then do that along all the rest of the vent and it will pull out. Test drive with the vent out and the noise should stop, then you know you found the issue. The service manual says you can fix this with silicon spray or foam tape around the rim of the vent. It must be a common issue because they are specific about the fix when they aren't about others.
Sent from my D6603 using Tapatalk