If you are experiencing a rattle or rubbing noise which seems to come from the seat belt height adjuster (or from the same location on the passenger side), it could possibly be the infamous B-pillar rattle. 

In 99.5, Audi changed the exterior B-pillar cover (outlined in picture) from flat black to painted glossy black. In doing so, the panel's fitment changed slightly (for the worse). On many cars, going over small bumps causes enough chassis flex for the B-pillar cover to rub against the rubber weather stripping on the roofline. 

One way to determine if this is your rattle is to remove the B-pillar covers and drive around for a while (on a road which usually caused the noise). Remember to do this on a dry day as it may compromise your car's water seal. Four bolts hold it in place. They are all Torx-20, sometimes called "star hex" if you're not familiar with them. First, open both doors on the side you are working on. You will see a plastic panel at the bottom of the B-pillar cover. Simply pull up and out, it will pop off. Two bolts are located there. The other two bolts are about four inches from the roofline, one on each side of the B-pillar cover. Pull back the rubber door seal with your fingers to find them. Once you have the four bolts off, the metal cover should pull straight off. 

Take the test drive. If the noise/rattle is not gone, sorry, this wasn't your problem. If it IS gone, you must reinstall the B-pillar covers in such a way as to not cause the noise. No amount of foam on the inside will prevent the noise, as it is coming from metal to rubber contact at the roofline. Simply reinstall the cover 1/8" lower than it was previously, there is enough play for that. There will be a very small panel gap at the roofline, which to me is a tiny price to pay in exchange for "no more B-pillar rattles!" 

I had the rattles on both sides, and after trying many other suggestions, the above is what I did. Now, almost one year later, I have never had the problem recur.