You likely have the Apple External Video Connector (M4099LL/A) installed in your computer. The ribbon cable of this connector can sometimes rise up and contact the exposed blades of the fan assembly, which causes this noise.
To resolve this issue, press the ribbon cable down flat, or (if the cable is long enough), fold it as recommended in the External Video Connector User's Guide.
Note that this symptom can occur in any of the following computers:
* Power Macintosh 5200/75 LC
* Power Macintosh 5300/100 LC
* Macintosh Performa 5200 series
* Macintosh Performa 5300 series
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