Alternator is a good guess, or fan bearing (if clutched), but I would remove the belt and see how all the bearings feel by hand. With a new tensioner, the added tension you likely have now could have pushed a marginal bearing over the edge.

But try first: if you manually clutch the fan IN by shorting the switch contacts, the sound will vanish if the fan bearing is the source.

Steve

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'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7