Just another thought on this problem. If the tensioner is operated by oil pressure and the oil pressure normally drops when the RPM drops, could the sound you hear be caused by too low an oil pressure at idle? What does you oil pressure guage show at low RPM?

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1986 300E
1993 Volvo 940 Wagon
1984 Volvo 240 (daughter's)
1976 Dodge 1/2T pickup, gas hog