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.
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