Whistle/buzz in heating/cooling system

[jaws38]

My 2000 Crown Vic makes a buzzing sound when the climate control system is either off or any setting besides Defrost or Floor/Defrost is on.

To drive without hearing this buzz you have to have Defrost or Floor/Defrost on.

When you accelerate steadily the buzz goes away, so it seems to be related to the vacuum. Also it doesn't start until the engine has been operating for a while.

The Ford dealer couldn't reproduce it, probably because they didn't test-drive it long enough or they just idled in their garage.

Anyone ever heard of this or similar?