Tracking down noise in 97 Taurus

97 Taurus ~120,000 miles, 3.0 vulcan engine.
At idle in gear, the car makes a "buzzing" noise from the front of the car, sounds like a heat shield slapping around, but I can't see anything
obviously loose. If you take it out of gear or touch the gas pedal, it goes away. Also, if I put the AC on, the noise changes to a random metallic clang/tap along with the buzzing, like hitting a wrench against a piece of metal. Again, only at idle in gear, and it goes away as soon as I hit the gas a touch. The AC functions fine, IE blows very cold air.
I haven't ruled out the heat shield issue, but the totally different (and much louder) noise when the AC compressor engages makes me wonder.
Any guesses?
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Turns out the noise is the serpentine belt slapping around at idle, it's not under enough tension. Would this be due to belt stretching generally, or am I likely to need to replace the tensioner? (ie, is the tensioner failing a common problem). The belt looks fairly old (maybe even original, I haven't had this car that long).

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"Teknical" wrote

Spring-loaded tensioner? Should be indicator marks on it showing when the belt has stretched enough to be replaced.
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