belt tensioner question

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On M50 engines, do belt tensioners tend to wear out at certain points? My 91 525 has 143,000 miles on it. There is a metallic whirring sound from the front of the engine and I'm guessing/hoping that the noise is as simple as a tensioner gone bad. I don't think that the sound is internal engine noise.
Any ideas?

TIA

edit: forgot to mention that I replaced the old belt with a new.

My '95 M50 had a noisy idler pulley at 85,000 miles, this is the highest one you see. Do you have one of those auto stethascope (sp?) gizmos where you can isolate engine noises? It could help to pin down which one is whining.

Good luck, John 1995 525im

Once it's isolated, how do you go about fixing it?