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Polo 9N 1.4 16V 75bhp.
I 've had new cam belts and tensioners fitted today, and the engine now makes a very noticable whirring noise when revved and also noticable as a steady whine when running at say 30mph.
Previously it was very quiet. Maybe it's the new belts/tensioners causing this and will quieten down after a time or maybe it's the wide flat belt driving the alternator, aircon pump etc. that's been tensioned incorrectly. Just wondering if anyone else has had this problem?

take it straight back and have the garage look at it. You don't want it over tensioned or it'll snap.
My mechanic will not tension a belt at all unless its brand new. Just too much risk with tensioning belts. Make sure its done right in the first place.

Thanks. Are you suggesting that it is not the cam belt, but the alternator belt that might be the problem? I could have a look myself at the alternator belt tension if I knew where to check and how much play etc.

As stated already - take it straight back, which ever of the belt it is.
If it is the fan belt then its a quick fix by the garage - if its the cam belt and they've done it wrong then you could end up in the brown stuff if they don't correct it sharpish.