audi a4 avant 2.5 B5.5 quattro - weird engine speed related noise

audi2.5tdi

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just picked up a b5.5 avant 2.5 quattro for the winter, its had a timing belt done 35k miles ago

Ive got an engine speed related noise, sounds like its coming from the timing belt area, almost the sort of noise you would expect from a tight belt, like a whistly whine, goes up and down with engine speed

could it be the aux belt or likely to be timing belt? or is there something else in that area, that could make a similar noise, pulley, fan etc etc
 
failing or just noisy, whats the best way to identify the culprit