Serpentine belt 06 jetta tdi

Very possible: bad alternator pulley clutch. Remove the serpentine belt and stick a soft wood stick to jam the alt fan. The pulley should only rotate 1 way. If it rotates 2 ways replace it. You need a special tool to loosen the pulley which can be purchased at metalnerd.com.

If not, bad tensioner?

Also, post moved to mk5 technical section, and title edited to add year.
welcometomyturbodies

No, just the alt pulley. Possibly a bad tensioner. Otherwise, is your crank pulley off? What service did it recently have that may have affected it?

AH, ok, I misunderstood. Your alternator pulley is definitely bad than.

Take a look through your owner's manual and the 2005-2006 new buying checklist in the FAQ section for other stuff you may not be aware of.