Could be the exhaust.
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I had a funny noise that I would have described more as a whine.
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Only happened on idle.
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Checked alternator as thought it was that area and couldn't find it.
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Was about to give up when I'd thought I'd try the old Ed China trick of sticking a bit of tissue on the end of a stick and shoving it around the engine bay.
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Low and behold I found an exhaust leak from my cat to downpipe.
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Took it apart and found a weird gasket that wasn't right.
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Bought a Toyota OEM gasket and installed it and noise now gone.
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