Common complaint. It's even addressed in the owner's manual. It states that the noise should stop after a few miles/minutes. It also says don't rev beyond 3k until it does. The cam lobe or lifter face can be damaged. This topic's been covered here frequently and a search will turn up lots of hits.

Basically, the valves cannot be adjusted as the lifters are hydraulic. To troubleshoot follow these steps in order and proceed to the next until the noise stops.

1. Check your oil level. Make sure it's correct.
2. Change your oil and filter. Use the grade suggested by the manual. Probably 10W30 this time of year depending on ambient temps. Go thinner and a leaky lifter won't hold pressure. Go heavier and a partially blocked lifter or lifter gallery will prevent the lifter from pumping up properly and taking up clearance.
3. Use an engine flush product as supplied by Amsoil or BG and change your oil and filter again.
4. Replace (or have it done) the 0-rings inside the oil pan that are on the oil pickup tube and filter galleries. They deteriorate and allow the pump to suck a little air causing oil cavitation. Result; noisy lifters.
5. Replace lifter(s). They're available relatively cheap online but replacement is a very labor intensive job require you have or make special tools.

Mine stopped making noise after step 3. You wouldn't believe the toxic crap that came out of the engine. That was 8,000 miles ago.

Good luck.
--
Erwin in Memphis, '95 855t