I solved our seatbelt rattle with Velcro some time ago and it works.

Look to see where the seat belt is tapping against the plastic B pillar. Put a 2 inch piece of fuzzy Velcro vertically (so it works no matter where the buckle is located on the belt).

That's it - no more rattle!

 

I run with my windows down all the time and never have a problem. After seeing this thread I took a look at mine.I think the key to my success is that the passenger seat is all the way back, and the "stopper" on the belt for where the buckle rests happens to be right at seat bolster level of the passenger seatback. So in effect, the buckle is resting against the passenger seat, and does not seem to budge.

 

I tried the velcro for the last few days. It cured one problem but created another.

The buckle doesn't knock around anymore but the seatbelt vibrates in the wind... I'm not sure which is worse.

So, I realized that if you turn the seatbelt 90 degrees to the right, it doesn't flap, vibrate or anything.

Works for me.

Mark