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Hello, maybe I can help. What kind of car are we talking about? Also, was it in good running condition when you first brought it in for the timing belt replacement?
Ok. It sounds to me like the mechanic who did the job did not time the engine right when putting it back together. This most likely caused internal damage to the engine which is why there is a noise now. When it comes down to it if it was running before with no noise and now it has a noise it's because they probably did something wrong. Tensioners don't just go bad like that usually. At least I've never seen it happen.
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