A fan mounted to the chassis inside the Macintosh Plus.
https://cdn.masto.host/retrochatonline/media_attachments/files/111/500/801/596/152/563/original/55e859b454f2bbc4.png
As soon as I turned it on, I could hear this.
This looks like part of a very professional mod. I removed the power cable, but saw the fan cable seemed to be trailing off elsewhere...
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