It's usually caused by a bad alternator pulley apparently, they're clutched and can start to fail which causes the tensioner to bounce about. If you don't fix it in time it can break the tensioner mounting too so might be worth looking into sooner rather than later. I'm sure there's a faq on here somewhere, I'll see if I can find it and post back
The only other possibility is the crankshaft pulley. But yes, the clutched alternator pulley is highly suspect. Put a wood stick in the fan to counterhold the shaft and rotate the pulley by hand (motor off, belt off). it should only rotate 1 way.
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