Why does the my Audi A4 make a strange rattling noise when I start the engine?

I've noticed that when I start up my Audi A4, there's a strange rattling noise coming from the engine. It goes away after a few seconds, and only happens when the car has been sitting for a few hours. What could be causing this?

The rattling noise is most likely caused by a failing timing chain tensioner. It's a common issue in older Audi A4s and can be replaced for a relatively low cost.

Another possible cause is a worn out serpentine belt. As the belt wears down, it can start to make strange noises, especially during start up. Consider replacing the belt if it hasn't been done recently.

I had a similar issue with my Audi A4 and it turned out to be a faulty engine mount. The rattling noise was caused by the engine shaking and hitting against the mount. Check your mounts and replace if necessary.