Sounds like the aspirator fan. It sucks air from a foam tube, which in turn goes up the "A" pillar and into the courtesy light assembly near the rear-view mirror. There is a thermistor under a grill in the courtesy light assembly which provides feedback to the climate control system. The purpose of the aspirator is to ensure a constant sampling of cabin air temperature.

If you shut off your climate control while the buzzing is occuring, the buzzing should stop if it's the aspirator.

If it is the aspirator, it eventually should be fixed or replaced. If it isn't working properly, the climate control system tends to act erratically.

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Steve
'93 400E