| The auxiliary belt which drives the alternator (and more?) is loose or worn. A competent mechanic should be able to replace or retension it in 5 minutes.   Demand they fix it... It could be that the alternaor bearings are on the way out, or the water pump is seizing. (unlikely but.£££s)   It's their problem.   the garage I bought it from suggests that once it's had a good run it should stop doing this   The phrase for this is "male cow excrement" or more colloquially.. 
                                Edited by madf on 26/04/2012 at 18:03
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