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If you hit the "O" button did the noises stop?
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Gilly:

What is usually wrong if the noises does not stop once the "O" button is hit?

The ACC on my 87 300TD has some issues. If I hit the "O" button, it still blows a minimal amount of air unless I keep the "O" button depressed. After I release, it will blow some air again after 5 - 10 seconds.

Thanks.