almost certainly a failed check strap, the two major components of the check strap are fastened together using two large pop rivets (6mm Diameter)
These fail over time due to metal fatigue, once one goes the other fails very quickly. Once the two parts are no longer mechanically fastened together when the door is open and closed there is a very noticeable clicking noise, in addition to this the check strap rubs heavily on the speaker housing and the internal door removable skin, both of which will be damaged irreparably if the failed check strap is not replaced.
You can buy a check strap from Audi at around £38 ea and these will fail again in the future as the original design remains the same, or you can buy one of my refurbished check straps, which have stainless steel bolts replacing the pop rivets and brass rollers replacing the plastic rollers that Audi use
As long as some lubrication is applied a couple of times a year I see no reason why these check straps will not last for many many years
They are £25ea plus £3 postage and a £20 deposit which is returned when I get you old check strap back, so better and 2/3rd of the price of an original one
Drop me a PM if you want one (or two if you decide to replace the passenger side one as well, you can get two for the same postage posts)
They are a 15 min job to change, remove door card, drift out the pin in the A pillar, it comes out by tapping it down unless some previous owners has refitted the pin upside down, and two 6mm (10mm across flats) nuts hold the check strap to the door. Refitting is a revers of the above.
There are three torx screws hold the door card in place, two under the grab handle and one hidden behind the tweeter grill
You will find a guide on here for door card removal
cheers,
Paul