The converter fan in a WFCO unit runs, irrespective of battery status, each time it's plugged into shore power. Short period, maybe a test cycle. After the test cycle, the fan runs only as temp algorithm related to charge profile calls for the fan.
I had a similar experience when a loose wire was touching the fan blades. It was only noticeable when initially plugged in.
The converter fan sound can be verified by switching on 12v loads such as lighting when on shore power. The more 12v load, the higher the fan speed...and the fan becomes more noticeable as it speeds up.
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